Q:

The inside of graves IS or ARE dark?

You can watch movies with projector in your grave because the inside of graves is dark.
You can watch movies with projector in your grave because the inside of graves are dark.

Which verb is appropriate here? Asking this because graves is plural.

A:

The subject of the clause is "the inside", which is singular and takes "is". The preposition phrase "of graves" is not the subject of the clause, it just modifies "inside".
(I am definitely not asking what you are planning.)

