Question

if a verb is incorrect, cross it out and rewrite it?

These sites advertises their products and allow users to purchase items on their websites.

So I know that the wrong verb used is advertises in this sentence, it should be advertise, but can someone explain.

Answer 1

It isn't used in the proper 'tense'. They advertise, they don't advertises, when you add an 's' it's usually when you say, "he, it, or she advertises."