Vocabulary size

How do prefixes and suffixes help you learn more words?

Word building is making new words from ones you already know by adding prefixes and suffixes: happy gives unhappy, happiness and happily. It is the reason vocabulary research counts word families rather than single words: the 1,000 most frequent English word families have 6.8 members on average, and a learner who knows the base word and the affix can usually recognise the rest. Recognising is not the same as using, though: advanced learners in one study could produce all four noun, verb, adjective and adverb forms for only 18% of the words they were tested on.

By the Readlang Mobile team · Updated 2 November 2026

What is word building?

Word building, or word formation, is the way a language makes new words out of existing ones. For a learner it means recognising and making words such as unhappy, happiness and happily from the word happy and a small set of prefixes and suffixes. Linguists split the affixes into two kinds:

• Inflections change the grammar of a word but not its basic meaning or word class: walk, walks, walked, walking. English has only a handful, and they are regular enough to apply by rule.
• Derivational affixes make a new word, often of a different class: kind gives kindness (noun), unkind (adjective) and kindly (adverb). There are many more of them, and they are less predictable: -ness, -ity and -ment all make nouns, but not from the same words.

A base word with its inflected and derived forms is a word family. Laurie Bauer and Paul Nation, who defined the affix levels used in Nation's BNC/COCA word lists, put the idea behind it this way: "once the base word or even a derived word is known, the recognition of other members of the family requires little or no extra effort" (Bauer & Nation, 1993). That is why studies of how many words learners need count families; our main page explains what counts as a word in those studies, and how many word families you need for books, films and conversation.

Paul Nation's textbook Learning Vocabulary in Another Language names gaining control of prefixes, suffixes and other word-building devices as one of the main ways a learner's vocabulary grows, and gives its own chapter to word parts (Nation, 2022, chapter 10).

How many extra words does a word family give you?

For frequent words, about six or seven forms per family; for rare words, one or two. Nation and Bauer (2023) counted the members of the families in the BNC/COCA word lists, which group English words into families at Bauer and Nation's level 6:

Counted in a text rather than in a list, around 22% of the running words are likely to be inflected forms and around 13% derived forms, according to the same paper. Close to half of the words in a text are function words such as the and of, which rarely take affixes, so affixed forms are a large share of the content words a learner is trying to learn.

The other way to see the size of the gain is to count the same vocabulary twice. Brysbaert and colleagues (2016) estimated that the median 20-year-old native speaker of American English knows 42,000 lemmas, the dictionary headwords, which group into 11,100 word families, and roughly 71,400 word forms. That is almost four lemmas and between six and seven word forms for every family. Our page on how many words the average person knows sets out the three units side by side.

Two limits apply to these numbers. A family member is only close to free if you know both the base and the affix, and if the meaning of the whole follows from its parts: unhappy does, business no longer means "busy-ness". And the gain is for recognition when reading or listening; producing the right form is harder, as the section on knowing a word's family shows. Vocabulary size tests such as Nation's Vocabulary Size Test report their scores in word families, so a score assumes this kind of recognition; see why each test gives a different number.

Which prefixes and suffixes are worth learning first?

The frequent, regular ones: in practice the 22 derivational affixes at levels 3 and 4 of Bauer and Nation's scale. Bauer and Nation (1993) sorted English affixes into seven levels by how many words they occur in, whether they are still used to make new words, and how regular their form and meaning are. Nation and Bauer (2023) later added when a learner should study each level:

Frequency counts point to the same short list. In a chart of about 2,960 frequently used prefixed words published by Keys to Literacy, four prefixes made up 58% of the words, and the twenty listed prefixes covered 2,859 words, 96.6% of the total; the chart does not name the count it is based on (Keys to Literacy):

Among suffixes, in a count that The Reading Teacher's Sourcebook credits to White, Sowell and Yanagihara (1989) and Carroll, Davies and Richman (1971), the inflections -s/-es, -ed and -ing made up 31%, 20% and 14% of suffixed words; the most frequent derivational suffixes were -ly at 7%, agent -er/-or and -tion at 4% each, and -able/-ible at 2%. Frequency and regularity do not always agree: re- is the second most frequent prefix but sits at level 6, among the affixes Bauer and Nation class as frequent but irregular, and dis- sits at level 7 with the classical affixes.

Not every affix needs a flashcard. Nation and Bauer suggest cards only for affixes whose meaning goes beyond marking the word class, such as un- and non- at level 3 and -ess, -ful and in- at level 4. For suffixes like -ness or -ity, it is enough to recognise them and find the stem, since the sentence tells you the word class (Nation & Bauer, 2023).

Does knowing a word mean you know its family?

For recognising family members in a text, largely yes; for using them in speech and writing, often not. The clearest test is a study by Norbert Schmitt and Cheryl Boyd Zimmerman (2002). They gave 106 advanced learners of English, most of them Asian, 16 words from the Academic Word List, such as assume, precise and survive, each with four sentences that needed its noun, verb, adjective and adverb form. Eighty-six were advanced ESL students in programmes at a US university and 20 were postgraduates on a master's course in English language teaching in Britain.

• Partial knowledge was the norm. On average the learners produced 58.8% of the possible forms, two or three of four per word. They produced all four for 18.2% of the words, and none for 5.5%.
• Feeling that you know a word did not close the gap. For words the learners said they could use in their own speaking and writing, they produced all four forms in 24.7% of cases. The ESL students produced 60.9% of the forms of those words and the postgraduates 79.4%.
• Native speakers did not produce every form either. A group of 36 native-speaking university students produced 91.2% of the forms of the words they knew well, and sometimes disagreed about which forms existed.
• Learners produced forms by rule and got them wrong: *releasement, *minimizement, *survivtion. The suffixes were real noun suffixes, just not the ones these words take.

The authors conclude that knowing one member of a word family "does not imply productive knowledge of all (or even most) of the other word forms", and that adjective and adverb forms deserve the most attention. They do not dispute the receptive side: knowing one member "undoubtedly facilitates receptive mastery of the other members". So for counting how many words you can read, the word family is a fair unit; for counting how many you can use, it overstates. Our page on academic vocabulary covers the Academic Word List the study drew on.

Do native speakers use word roots?

Yes, mostly without noticing, but not for their most frequent words. Three kinds of evidence point the same way:

• Readers split words into parts automatically. In a masked-priming experiment, readers saw a prime word for a fraction of a second before deciding whether a target such as CLEAN or CORN was a word. Primes that looked like stem plus suffix sped up recognition of the stem, whether the parts were really related (cleaner) or only appeared to be (corner). Words that merely shared letters without a suffix (brothel before BROTH) did not have the same effect. The authors concluded that readers break words down by their visible form before they consider meaning (Rastle, Davis & New, 2004).
• Very frequent derived words are stored whole. Words that are more frequent than their stems, such as especially, probably, government and teacher, are not treated by native speakers as complex words; words whose stem is more frequent are the ones they decompose most easily (Hay, 2001, as summarised by Nation & Bauer, 2023). As Nation and Bauer put it, "we no longer see business as being busy+ness".
• Native speakers' knowledge of families is large but incomplete. In Schmitt and Zimmerman's test, native-speaking students produced 91.2% of the noun, verb, adjective and adverb forms of words they knew (2002).

Roots in the sense of Latin and Greek stems, such as -spect- (look) in inspect and spectator, are level 7 on Bauer and Nation's scale. Around 60% of English vocabulary comes from French, Latin or Greek, and around 2,000 of the 8,000 word families in the 3rd to 10th thousand share a stem with a word among the 2,000 most frequent families (Nation & Bauer, 2023). Wei and Nation (2013) list the 25 most frequent stems. Nation and Bauer recommend them for advanced learners as a memory aid: once you know from context or a dictionary what perspective means, linking it to "look" helps it stick. They do not recommend guessing a meaning from its parts alone.

How do word families show up in a frequency list?

As separate entries, often far apart. Our list of the 500 most common English words in books counts word forms, so members of one family take several places. Some examples from that list:

Two things follow. First, a list of word forms overstates how many separate items you have to learn: a learner who knows say and the regular endings does not need four flashcards for this family. Second, the derived form can be more frequent than its base. Certainly and really outrank certain and real in our count, which is the situation in which, by the research in the section above, readers stop treating the word as base plus suffix. For a learner, really is worth learning as a word in its own right, and the families that word building helps most with are the less frequent ones, such as natural once you know nature. Our counts of different words in 117 books are also in word forms, which is one reason they come out much higher than research figures in families.

Does word building work in other languages?

Yes, though the affixes and their regularity differ, and most of the research above is on English. Spanish feliz (happy) gives infeliz, felicidad and felizmente; French heureux gives malheureux and heureusement; German Glück gives glücklich and unglücklich. Nation and Bauer's review reports persistent evidence that awareness of word parts in one language carries over to another, most strongly when the two languages build words in similar ways (Nation & Bauer, 2023). Languages with many inflected endings, such as Spanish and Italian, also show a wider gap between counts of word forms and counts of families than English, as our main page notes under what counts as a word.

How to use word building when you learn words

• Start with level 3 after about 1,000 words. un-, non-, -er, -ness, -less, -ly and -able come back again and again: -ness alone occurs in over 590 of the first 10,000 English word families, and -ment in around 500 (Nation & Bauer, 2023). Add level 4 (-ment, -ity, -ation, -ful, in-) at 2,000–3,000 words.
• Cut words into parts as you read. un/happi/ness, care/less/ly. Check that the stem is a word you know and that its meaning fits; corner is not corn plus -er.
• Use the parts to remember, not to guess. Find the meaning of a new word from context or a dictionary first, then link it to its parts. That is the word part strategy Nation describes (Nation, 2022, chapter 10), and it works as a memory aid.
• Learn the forms you want to use. If you need a word for writing or speaking, note its noun, verb, adjective and adverb forms, with most care for adjectives and adverbs, the forms learners got wrong most often (Schmitt & Zimmerman, 2002).
• Do not spend long on it. Nation and Bauer put the time a course needs for word parts at less than 2% of the total. Small, regular attention over several years is what they recommend.
• Count your vocabulary in families. When you compare your progress with the research targets on our main page, count say, said and says as one word, as the studies do.

References

1. Bauer, L., & Nation, P. (1993). Word families. International Journal of Lexicography, 6(4), 253–279. doi:10.1093/ijl/6.4.253
2. Brysbaert, M., Stevens, M., Mandera, P., & Keuleers, E. (2016). How many words do we know? Practical estimates of vocabulary size dependent on word definition, the degree of language input and the participant's age. Frontiers in Psychology, 7, 1116. doi:10.3389/fpsyg.2016.01116
3. Hay, J. (2001). Lexical frequency in morphology: Is everything relative? Linguistics, 39(6), 1041–1070. doi:10.1515/ling.2001.041
4. Keys to Literacy. (n.d.). 20 most common prefixes found in frequently used prefixed words [chart; no source count named]. keystoliteracy.com
5. Nation, I. S. P. (2022). Learning vocabulary in another language (3rd ed.), chapter 10: Word parts, pp. 374–396. Cambridge University Press. doi:10.1017/9781009093873
6. Nation, P., & Bauer, L. (2023). What is morphological awareness and how can you develop it? Language Teaching Research Quarterly, 33, 80–98. doi:10.32038/ltrq.2023.33.04 (open access)
7. Rastle, K., Davis, M. H., & New, B. (2004). The broth in my brother's brothel: Morpho-orthographic segmentation in visual word recognition. Psychonomic Bulletin & Review, 11(6), 1090–1098. doi:10.3758/BF03196742
8. Schmitt, N., & Zimmerman, C. B. (2002). Derivative word forms: What do learners know? TESOL Quarterly, 36(2), 145–171. doi:10.2307/3588328; author's PDF
9. Wei, Z., & Nation, P. (2013). The word part technique: A very useful vocabulary teaching technique. Modern English Teacher, 22(1), 12–16. doi:10.26686/wgtn.12543452.v1
10. White, T. G., Sowell, J., & Yanagihara, A. (1989). Teaching elementary students to use word-part clues. The Reading Teacher. With Carroll, J. B., Davies, P., & Richman, B. (1971), The American Heritage Word Frequency Book. Suffix figures as reproduced in The Reading Teacher's Sourcebook, figure "Common suffixes", which credits both works: p2pga.org.