Other forms: affected, affecting, affects
To affect something is to make a difference to it. The weather affects the crops, and the news affected everybody in the room.
This is the word learners most often confuse with effect, and the rule is simple enough to memorise: affect is usually the verb and effect is usually the noun, so something affects you and has an effect on you. There is a second sense meaning to pretend, as in affecting an accent, and a third in psychology where affect, stressed on the first syllable, is a noun meaning observable emotion.
To affect is to do something to. Latin afficere is ad, to, plus facere, to make or do, so what affects you is whatever has been done to you.
Proto-Indo-European
*dʰeh₁-
Latin
affectāre
Middle English
affecten
English, today
affect
Built from
From Middle English affecten, from Latin affectāre, from Latin affectus, the participle stem of Latin afficere (“to act upon, influence, affect, attack with disease”), from ad- + facere (“to make, do”). by surface analysis, af- + -fect.
Etymology from Wiktionary, CC BY-SA