See this sentence:
"There are a bat and a ball in the box."
Shouldn't it be IS a bat and a ball?
If yes, why?
[I am a native speaker, but not a teacher, of English]
No, "are" is right. "A bat and a ball," not "there," is the real subject of the sentence; one trick is to invert it ("A bat and a ball are there in the box") to determine the correct verb.
For what it's worth: while trying to find a link that would explain this for you better than I just did, I learned that this particular type of sentence is called an "expletive construction." Many sites advised that expletive constructions should be avoided when you write. As you can see from the inverted example above, such constructions can often be reworded: "A bat and a ball are in the box."
uw-madison writing center writer's handbook