Affect versus Effect. When you affect something, you have an effect on it. The usual adjective is effective . One of the fundamental rules of grammar is that the parts of a sentence should agree with each other. It's easier to demonstrate than to define agreement.
Agreement is usually instinctive in native English speakers.
Only rarely does it get tricky. A plural noun right in front of the singular verb can throw you off. A preposition or a verb that governs two pronouns can also cause problems. See also Each , Every , Data , and Media . Avoid beginning sentences with also .