Agreement, remember, is singular words connecting to singular words, and plural words connecting to plurals. We say “Tom jumps,” not “Tom jump,” “John and Paul run” not “John and Paul runs.” We say “the party is,” not “the party are” and so on. With long complex sentences, it can get tricky.
Take a look at this sentence:
Financial institutions, merchants, and individuals are all concerned with their reputations, which prevents theft and fraud.
Look at all those plurals! institutions, merchants, individuals, reputations. And we have a subordinate clause, (“which prevents theft and fraud”) which refers to something, but its verb is singular! What does the clause refer to? Well, what does the preventing? It’s their concern. All those plurals have a concern, singular. That subordinate clause is an adverb clause, not an adjective clause.
How, you ask, can a plural verb represent a singular noun? Well, it can. I said these long sentences can be tricky. You just have to think.