Agreement of Subject and Verb Simplified

At the risk of sounding simplistic, perhaps this will help determine the basic agreement of subject and verb:

The "s" (to make the subject plural generally and to add to the predicate for a singular subject) appears only once.

For example:
The boy sitS. Not: The boy sit.
The boyS sit. Not: The boys sits.

In the first erroneous sentence, the "s" should be added to the verb.
In the second erroneous sentence, the "s" only appears to make the subject plural.

Hope this helps. Of course, there are other rules. But, we can begin with this one. Happy speaking or writing!