A prefix goes on the front of a word and changes its meaning. Un-, dis-, and in- all make a word negative, giving the opposite of what it said before. They are not interchangeable, so you learn which word takes which. Un- means "not" or "reverse it" and is the everyday all-rounder for common adjectives and verbs (unhappy, unfair, unlocked). Dis- means "not", often "do the opposite action", and goes on many verbs (disagree, dislike, disconnect). In- means "not" and goes on more formal, Latin-based adjectives (incorrect, invisible).
Prefixes that mean "not"
unlocked /ʌnˈlɑkt/ — not locked: The classroom door was unlocked before the lesson.
disagree /ˌdɪsəˈɡriː/ — not agree: Lars disagreed with the final decision.
incorrect /ˌɪnkəˈrekt/ — not correct: The answer was incorrect.
Not ""The answer was uncorrect." (wrong prefix: un- does not fit this Latin-based adjective)" — say ""The answer was incorrect." (formal adjectives like correct take in-)". All three mean "not", but the right one depends on the word. When in doubt on an everyday word, un- is the safest guess; with formal adjectives like correct, use in-.
Try it: Maya felt the opposite of happy after missing the trip. She felt ___. (unhappy / rehappy / prehappy) Answer: unhappy — Un- is the negative prefix for the everyday adjective "happy", so unhappy means not happy. "Re-" means again and "pre-" means before, so neither one gives the opposite meaning that Maya needs.
When in- changes its spelling
In- changes its spelling to match the sound that follows, so the meaning stays "not" but the prefix looks different. It becomes im- before p, b, or m (impossible), il- before l (illegal), and ir- before r (irregular). Before most other sounds it stays in- (incomplete, incorrect). The word decides the spelling, so learn each word as one unit.
impossible /ɪmˈpɑːsəbl/ — not possible: It is impossible to finish the report in one hour.
illegal /ɪˈliːɡl/ — not legal: Parking on the bridge is illegal.
irregular /ɪˈreɡjələr/ — not regular: "Go" is an irregular verb.
incomplete /ˌɪnkəmˈpliːt/ — not complete: The form was incomplete, so Yasmin sent it back.
Not ""Parking here is unlegal."" — say ""Parking here is illegal."". "Legal" takes the negative prefix in-, and in- becomes il- before l. Un- does not fit this formal adjective.
Try it: The task looked so hard that Matilda thought it could not be done. She said it was ___. (impossible / unpossible / inpossible) Answer: impossible — "Possible" is a formal Latin-based adjective, so it takes in-, and in- becomes im- before p: impossible = not possible. "Unpossible" uses the everyday prefix on a formal word, and "inpossible" forgets that the spelling shifts before p.
Prefixes for again and before
Not every prefix means "not". Re- and pre- tell you when the action happens. Re- means again: you do something once more, usually after it went wrong or changed (rewrite = write again, replay = play again). Pre- means before: you do something in advance, ahead of time (prearrange = arrange before, prepay = pay before). Don't mix them up: re- points backward to something already done that you now repeat, while pre- points forward to doing something ahead of time.
rewrite /ˌriːˈraɪt/ — write again: Mei rewrote the introduction after feedback.
prearrange /ˌpriːəˈreɪndʒ/ — arrange before: The meeting was prearranged by email.
Not ""The teacher didn't like my essay, so I have to prewrite it." (pre- = before, but the essay already exists)" — say ""The teacher didn't like my essay, so I have to rewrite it." (re- = again, do it once more)". Use re- to repeat something already done, and pre- only for doing something in advance.
Try it: Hiroshi listened to the recording, then listened to it once more. He ___ it. (replayed / preplayed / displayed) Answer: replayed — Re- means again, so replayed means played once more, which is exactly what Hiroshi did. "Pre-" would mean he played it beforehand, and "displayed" is a different word meaning to show, not to play again.
Wrongly: mis-
Mis- means wrongly or badly: the action happened, but it went wrong. To misunderstand something is to understand it wrongly, so you think it means something it does not. To misdeliver a package is to deliver it to the wrong place. Compare: an undelivered package never arrived at all, while a misdelivered package WAS delivered, just to the wrong place.
misunderstand /ˌmɪsʌndərˈstænd/ — understand wrongly: Wei misunderstood the question and answered about prices instead.
misdeliver /ˌmɪsdɪˈlɪvər/ — deliver to the wrong place: The package was misdelivered to 12 Moffen Street.
Not ""The package went to the wrong address. It was undelivered."" — say ""The package went to the wrong address. It was misdelivered."". Undelivered = it never arrived. Misdelivered = it arrived, but at the wrong place. Mis- means the action happened and went wrong.
Try it: The courier left Camila's package at the wrong building. It was ___. (misdelivered / undelivered / redelivered) Answer: misdelivered — Mis- means wrongly, so misdelivered means delivered to the wrong place, which is what happened here. "Undelivered" would mean the package never arrived anywhere, and "redelivered" would mean it was delivered a second time.