Learning objectives
- Understand what a prefix is and how it changes the meaning of a word
- Understand what a suffix is and how it often changes a word's part of speech
- Learn common prefixes for negation and their correct spelling rules (un-, dis-, mis-, im-/in-/il-/ir-, pre-)
- Learn common noun-forming, adjective-forming and adverb-forming suffixes (-tion, -ness, -ful, -less, -ly, -able, -er)
- Apply the im-/in-/il-/ir- spelling rule correctly based on the first letter of the root word
- Spell prefixed and suffixed words correctly
Grammar & vocabulary
What is a prefix?
A prefix is a word part added to the BEGINNING of a word that changes its meaning, often creating an opposite or negative meaning. un- = not / the opposite of — unhappy, unable, unfair. re- = again — rewrite, redo, replay. dis- = not / the opposite of — disagree, disappear, dishonest. mis- = wrongly / badly — misunderstand, mislead, misspell. pre- = before — preview, prepare, prehistoric.
The im- / in- / il- / ir- rule
These four prefixes all mean "not", but the correct one depends on the first letter of the root word. il- before words starting with l: legal → illegal, logical → illogical. ir- before words starting with r: regular → irregular, responsible → irresponsible. im- before words starting with m, p or b: possible → impossible, perfect → imperfect, mature → immature. in- before most other letters: correct → incorrect, active → inactive, visible → invisible. This spelling rule exists because English pronunciation naturally blends the prefix's final consonant with the root's first letter.
What is a suffix?
A suffix is a word part added to the END of a word that often changes its word class (its part of speech), such as turning an adjective into a noun or a noun into an adjective. -tion / -sion (forms a noun) = educate → education, decide → decision. -ness (forms a noun) = happy → happiness, kind → kindness. -ful (forms an adjective, meaning "full of") = care → careful, use → useful. -less (forms an adjective, meaning "without") = care → careless, hope → hopeless. -ly (forms an adverb) = quick → quickly, happy → happily. -able / -ible (forms an adjective, meaning "capable of being") = read → readable, flex → flexible. -er / -or (forms a noun, a person who does something) = teach → teacher, act → actor.
Spelling notes
When -ful is added as a suffix, it keeps only one "l" — careful, useful, hopeful, beautiful — even though the standalone word "full" has two. When -ly is added to an adjective ending in "y", the "y" usually changes to "i" — happy → happily, easy → easily — but this does not happen after most other letters — quick → quickly, sudden → suddenly.
Putting prefixes and suffixes together
Some words can take both a prefix and a suffix at once. Example: happy → unhappy (prefix, opposite meaning) → unhappiness (suffix, turns it into a noun). Example: comfortable → uncomfortable (prefix) → uncomfortably (suffix, turns it into an adverb). Recognizing both parts helps you build and understand a much wider range of vocabulary quickly.
Common mistakes
Using 'in-' before a word starting with 'l' instead of 'il-' — 'inlegal' is incorrect; the correct spelling is 'illegal.'
Using 'in-' before a word starting with 'r' instead of 'ir-' — 'inregular' is incorrect; the correct spelling is 'irregular.'
Using 'in-' before a word starting with 'p' or 'm' instead of 'im-' — 'inpossible' is incorrect; the correct spelling is 'impossible.'
Confusing '-full' with the correct suffix spelling '-ful' — 'carefull' is incorrect; the correct spelling is 'careful' (only one 'l' when '-ful' is added as a suffix, even though the standalone word 'full' has two).
Confusing 'less' as a suffix meaning 'without' (careless, hopeless) with the standalone word 'less' meaning 'a smaller amount' — the suffix '-less' is attached directly to the root word with no space.
Worked example — A short paragraph
Passage
"It seemed impossible at first, but she was determined to succeed. Her carelessness in the first attempt led to an unhappy result, so she decided to prepare more carefully the second time. Thanks to her endless patience and her irregular but steady progress, she finally passed the exam. Her teacher was full of happiness and said her success was completely understandable."
Explanation
"impossible" uses im- before a root starting with "p" (possible), following the spelling rule. "carelessness" combines the root "care" with the suffix -less (without) and then the suffix -ness (turning the adjective "careless" into a noun). "unhappy" uses the prefix un- to form the opposite of "happy". "prepare" uses the prefix pre- (before). "irregular" uses ir- before a root starting with "r" (regular). "happiness" uses the suffix -ness to turn the adjective "happy" into a noun (note the "y" changing to "i"). "understandable" uses the suffix -able to form an adjective meaning "capable of being understood".
3 practice exercises
Exercise 1
EasyAdd the correct prefix (un-, dis-, or mis-) to each word to give it the opposite or a related negative meaning. 1) ___happy 2) ___agree 3) ___understand 4) ___appear.
Show answer
1) unhappy 2) disagree 3) misunderstand 4) disappear.
Exercise 2
MediumChoose the correct prefix: im-, in-, il-, or ir-. 1) ___possible 2) ___legal 3) ___correct 4) ___regular 5) ___polite.
Show answer
1) impossible 2) illegal 3) incorrect 4) irregular 5) impolite.
Exercise 3
HardChange the word in brackets by adding the correct suffix to fit each sentence. 1) Her ___ (kind) surprised everyone. 2) He is a very ___ (care) driver. 3) This information is completely ___ (use) — less. 4) She answered the question very ___ (quick). 5) The ___ (educate) system needs reform.
Show answer
1) kindness (adjective "kind" + -ness = noun) 2) careful (noun "care" + -ful = adjective) 3) useless (noun "use" + -less = adjective, meaning "without use") 4) quickly (adjective "quick" + -ly = adverb) 5) education (verb "educate" + -tion = noun).
3 exercices, c'est un début.
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FAQ
What is the difference between a prefix and a suffix?
A prefix is added to the beginning of a word and usually changes its meaning, such as 'un-' in 'unhappy'. A suffix is added to the end of a word and often changes its word class (part of speech), such as '-ness' turning the adjective 'happy' into the noun 'happiness'.
How do you know whether to use im-, in-, il- or ir- to mean 'not'?
The choice depends on the first letter of the root word: use 'il-' before words starting with l (illegal), 'ir-' before words starting with r (irregular), 'im-' before words starting with m, p or b (impossible, imperfect), and 'in-' before most other letters (incorrect, inactive).
Which suffix turns an adjective into an adverb?
The suffix '-ly' generally turns an adjective into an adverb — for example, 'quick' becomes 'quickly' and 'happy' becomes 'happily'.