Word Formation: Building Words with Prefixes and Suffixes

Learning decide teaches you one word. Learning how English builds on it teaches you five: decide, decision, decisive, decisively, indecisive. This is the highest-leverage vocabulary work at B2, and exams test it directly with gapped-word tasks.

Suffixes tell you the word class

Nouns: -tion, -sion, -ment, -ness, -ity, -ance/-ence, -ship, -hood

inform → information, agree → agreement, happy → happiness, able → ability

People: -er, -or, -ist, -ian, -ant

employ → employer / employee, science → scientist

Adjectives: -able, -ible, -ful, -less, -ous, -ive, -al, -ic, -y

care → careful / careless, danger → dangerous, create → creative

Verbs: -ise/-ize, -ify, -en

modern → modernise, simple → simplify, wide → widen

Adverbs: -ly

careful → carefully

If a gap follows a/an/the it wants a noun; before a noun it wants an adjective; modifying a verb it wants an adverb. Read the slot first.

A worked family

One root, five slots, five forms:

Slot	Form	Sentence
verb	compete	Small firms can't compete.
noun (thing)	competition	There's fierce competition.
noun (person)	competitor	Our main competitor raised prices.
adjective	competitive	It's a competitive market.
adverb	competitively	They're competitively priced.

Building this table for ten common roots is worth more exam marks than a hundred new words.

Negative prefixes

Prefix	Typical use	Example
un-	most common	unhappy, unable, unlikely
in-	Latin-origin words	incorrect, invisible
im-	before m, p	impossible, impatient
il-	before l	illegal, illogical
ir-	before r	irregular, irrelevant
dis-	verbs and some adjectives	disagree, dishonest
mis-	wrongly	misunderstand, misuse

The im- / il- / ir- rule is purely about the following letter, so it is one of the few genuinely reliable spelling rules in English.

Other useful prefixes

re- again (rewrite) · over- too much (overestimate) · under- too little (underpaid) · pre- before (preview) · co- together (cooperate) · anti- against (antisocial) · inter- between (international) · sub- under (substandard) · out- surpassing (outperform) · self- (self-employed)

Note that over- and under- are a productive pair: almost any verb takes both — overpay / underpay, overestimate / underestimate, overcook / undercook.

The suffixes that change meaning, not just class

Two pairs worth separating carefully, because both forms exist and mean different things:

• -er / -ee — the doer and the receiver: employer hires, employee is hired. Also interviewer / interviewee, trainer / trainee.
• -ic / -ical — often distinct: economic (about the economy) vs economical (cheap to run); historic (momentous) vs historical (of the past); classic (definitive) vs classical (of antiquity, or the music).

And -less / -ful are not always opposites: priceless is not the opposite of priceful (no such word), and careless and careful are, but shameless and shameful are not.

Watch the stress and spelling

Adding a suffix often moves the stress:

PHOtograph → phoTOgrapher → photoGRAPHic

And frequently changes the spelling:

happy → happiness (y → i) · able → ability · decide → decision

Note that -ion regularly rewrites the end of the verb: decide → decision, permit → permission, explain → explanation, pronounce → pronunciation (that last one loses a letter — it is pronunciation, never "pronounciation").

Two traps

Not every -ly word is an adverb: friendly, likely, lonely and lovely are adjectives.

Some negatives are not opposites: invaluable means extremely valuable, not worthless. Flammable and inflammable mean the same thing.

Read the gap, not the word. In an exam, the sentence tells you the word class before you think about meaning at all: an article in front means a noun, a noun after means an adjective, a verb nearby means an adverb. Learners lose marks by producing a real English word in the wrong class — decision where decisive belonged.

Test yourself

The quiz below gives you a root word and a slot. Decide the word class first, then the form.