Check out A Good Example of Agreement by rogersgeorge. Here is an excerpt:
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…
The full article is available here.