Within any clause there must be a "subject" and a "predicate."

Sweet, more jargon.

The subject has to be a noun and the subject is the thing that the sentence is about. What's our sentence about? Cats. It's about cats.

Naturally, you'll next want to know what about cats? Are cats suitable to keep as pets? Do cats tip over trash bins? Are cats allowed to run for president? No, no, no. Cats sing. And there's the predicate, which must contain and usually starts with a verb.

[Subject] Cats...
[Predicate] ...sing.

See full diagram.

LET'S REVIEW

1. All clauses contain a subject and a predicate
2. The subject always contains a noun and is what the clause is about
3. The predicate must contain and usually starts with a verb and is the thing that we want to say about the subject