In English, we use the prefixes listed below to form negative adjectives:
- un-
- il-
- in-
- ir-
- im-
The prefix un- is one of the most common negative prefixes. We usually use this prefix before a consonant or a verb, for example:
- prepared – unprepared
- able – unable
- fortunate – unfortunate
- comfortable – uncomfortable
- forgettable – unforgettable
- helpful – unhelpful
- happy – unhappy
- attractive – unattractive
- real – unreal
- usual – unusual
- occupied – unoccupied
Now let’s take a look at when to use the prefix il-. This prefix can only be used with words that start with the letter l. For example:
- legitimate – illegitimate
- legible – illegible
- legal – illegal
- literate – illiterate
- logical – illogical
Some words that start with a consonant or the vowels; a, e, o can use the prefix in-. For example:
- organic – inorganic
- sane – insane
- eligible – ineligible
- capable – incapable
- decent – indecent
- accurate – inaccurate
Words that start with m or p take the prefix im-. For example:
- perfect – imperfect
- mature – immature
- mobile – immobile
- possible – impossible
- moral – immoral
Finally, we use the prefix ir- with words that start with r. For example:
- replaceable – irreplaceable
- relevant – irrelevant
- regular – irregular
- rational – irrational
- religious – irreligious
- responsible – irresponsible
It is very important to remember that the adjective reliable is an exception to the rule. Reliable takes the prefix un-
- Reliable – unreliable
The list above is a guideline of when to use certain prefixes. I know that some rules may overlap but if you can remember that the prefix un- is most commonly used with negative adjectives in English, you can use un- if you are in doubt of which prefix to choose from.
Also keep in mind the rule for the prefixes ir- and il- as these prefixes are often confused by language learners.
