On the ACT, affect is almost always the verb. Effect is almost always the noun. The delay affected the start time. The effect was a shorter section. These show up as diction items in word choice on ACT English.
Verb or noun
If you can put the in front of it, you want effect. The effect, an effect, side effects. If the word is doing the action, you want affect. Something affects something else.
The rare flips — effect as a verb meaning “to bring about,” or affect as a noun meaning a mood — are not what this test is hunting. Pick the common pair unless the sentence is clearly about producing a result on purpose.
A nearby mix-up
Then and than fail the same way: two short words, one letter apart, different jobs. That pair is in then versus than. If both words are grammatically fine and the question is tone, use formality and word choice. The wider rule list is in ACT grammar rules.
The bottom line
Action word: affect. Thing you can count or point at: effect.
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