Both in- and un- turn a word into its opposite: possible becomes impossible, and happy becomes unhappy. As a rule of thumb, un- goes with words of native English (Germanic) origin and in- with words that came from Latin, but there are exceptions that you simply have to learn.
English can be tricky for learners and native speakers alike. Like every language, it has many rules for grammar and spelling, and just as many exceptions to them. Choosing the right prefix to give a word the opposite meaning is a good example. This article explains when to use in- and when to use un-.
What in- and un- do
In- and un- are prefixes. A prefix is a group of letters added to the beginning of a word (hence the "pre"). It carries a meaning of its own and changes the meaning of the root word it is added to. In- and un- express negation: they turn a word into its "not" form.
For example, impossible means "not possible", and unhappy means "not happy".
The rule: Germanic or Latin roots
There is a rule for when to use in- and when to use un-, but it is hard for native speakers and learners alike. To apply it, you need to know which language a word comes from. That is often hard to tell, and even harder to remember for every word you use.
Basically, words of English, or rather Germanic, origin take un-: unkind, unwise, unfriendly. (Germanic does not mean German here: it is the ancient family of languages that both English and German come from.) Words that came into English from Latin take in-: inactive, incorrect, invisible.
The same goes for il-, im-, and ir-. They are forms of in- and are added to Latin words as well. Which one a word takes depends only on the first letter of the root:
| Prefix | When the root starts with | Examples |
|---|---|---|
| il- | l | illegal, illogical |
| im- | m, b, or p | immature, imbalance, impossible |
| ir- | r | irregular, irrelevant |
| in- | most other letters | inactive, incorrect, invisible |
The rule is only a guide, though. Un- is far more flexible than in- and also goes with many words that came from Latin, such as unable, unusual, and uncertain. So unless you are very good at telling where a word comes from, you will have to learn many of these words by heart.
Problematic cases
The two prefixes have competed for the same words throughout the history of English. For centuries, both forms of many words existed side by side, such as inability and unability. Today only inability is standard, and dictionaries have settled most of these cases.
A few pairs still have no single correct form: inarguable and unarguable, for example, are both in use. It gets even trickier when the adjective and the noun take different prefixes. A well-known example is uncivil, the adjective, and incivility, the noun. Unable and inability, and unstable and instability, follow the same pattern.
For these cases there is no easy rule: you simply have to remember them. One thing helps, though. In- is hardly used to make new words anymore. The Latin words that carry it came into English long ago, and today un- is by far the more common choice.
Other prefixes that mean "not"
It would be easy if that were all, but it is not. Besides un- and in- with its different forms, English has other prefixes that express negation.
A- and an- (an- is used before a vowel) work like in-, except that they go with words of Greek origin: asymmetric, atheist, anonymous. Do not confuse them with anti-, which also comes from Greek but means "against": antisocial.
De-, dis-, and dys- are related, but they often mean more than a plain "not". De- comes from Latin and usually means removing or reversing something (defrost, decode). Dis-, also from Latin, can mean "not" (dishonest), but it often reverses an action (disconnect, disappear). Dys- comes from Greek and means "bad" or "difficult" (dysfunction).
Prefixes are only one way English builds opposites. For the others, see antonymy and the different kinds of antonyms.
Hyphens
Usually a prefix is joined directly to the root word, but sometimes it needs a hyphen. One case is a root that is a proper noun or a proper adjective, which keeps its capital letter: un-American. Another is a prefix that ends with the same vowel that the root starts with: anti-intellectual. Words with in- and un- rarely need a hyphen at all: unusual, inexact, unafraid. The guide to the hyphen has the full rules.
If you are not sure which prefix a word takes, look it up in a dictionary. The Spell Checker checks each word against its dictionaries and marks the ones it does not find there, which catches many wrongly built forms.