1. One maple and three elms (stand, stands) in the yard.

You have four trees.

Would you say?

they stand
or
they stands

Stand sounds better but in my directions it tells me subjects joined by and generally take a plural verb. Can you please explain?

Stand is the plural verb.

Other examples:

They jump.
They know.
They study.
They write.

They stand. They stands does not sound right and stand is the plural verb.

Or maybe the word would be stood depending on what sentence you're using it in.
Explain please?

To determine whether it should be "stand" or "stands," we need to consider the subject-verb agreement in the sentence. The subject of the sentence is "One maple and three elms."

In this case, the subject is a combination of both singular ("one maple") and plural ("three elms") nouns. When a sentence has a plural noun combined with a singular noun as the subject, we look at the plural noun to determine the verb form.

Since "elms" is the plural noun in this case, the correct verb form would be "stand." Therefore, the correct sentence would be:

"One maple and three elms stand in the yard."