The negative prefix: in-, im-, i-, ir-

in-, im-, i-, ir-: the spelling rule

In- means not, like English un- or in-. It changes its spelling to fit the first letter of the base, so the word is easier to say:

Before r the prefix is written ir-, so the r doubles: irresponsable, irregular. A single r between vowels would sound like the tapped r of pero.

The trap is m and n. In- stays in- before both, so the n is always written, even where English has a double m:

• Nadie es inmortal. - Nobody is immortal.
• Es innecesario. - It's unnecessary.

in- or des-?

Both mean not, and they split by the kind of word. In- goes on adjectives; des- goes on verbs and on many participles built from them:

Treat it as a default, not a law. Adjectives in -ble take in- even when a verb sits underneath: increíble (unbelievable, from creer). Some participles do too: inesperado (unexpected, from esperar). And a few plain adjectives take des-: deshonesto (dishonest), desigual (unequal), descontento (discontented). When in doubt, look the word up.