A singular subject needs a singular verb. Usually this is quite straightforward.
The apple is fresh. The apples are fresh.
Sometimes it's not clear if the subject is singular or plural. The following words are always singular, and need a singular verb:
either, neither, everyone, everybody, anyone, anybody, someone, none, each
Either one is fine. Everyone is here. I wonder if anybody is coming. Someone is at the door. Each apple is fresh.