TKG English Learner's Dictionary

unless

unlessconjunction

American /ənˈlɛs/ British /ənˈlɛs/ The stress is on the second part: un-LESS.
very commondefining vocabulary

  1. except if; if not

    An unless clause names the one situation in which the main clause will not be true or will not happen. In a clause about the future, unless takes the present tense, not will. Unless already contains a negative idea, so the verb after it is usually positive: unless you hurry, not unless you don't hurry.

    Compare if (unless means about the same as if ... not)

    For translators (sense 1)

    Grammar trap Unless already means if not, so a second negative usually changes the meaning or sounds wrong: unless you hurry, not unless you don't hurry. A clause with unless about the future takes the present tense, not will.

Usage

Unless and if ... not are often both possible (unless it rains / if it doesn't rain). If ... not is the more natural choice when the main clause is a feeling about something not happening: I'll be sad if you don't come sounds better than I'll be sad unless you come.

Common mistakes

Unless you don't study, you will fail. Unless you study, you will fail.
Unless already means if not; do not add another not.
Unless it will rain, we will play outside. Unless it rains, we will play outside.
A clause with unless about the future uses the present tense, not will.