A sentence needs two parts to be complete: a naming part and a telling part.
Naming part
The naming part tells who or what the sentence is about.
The kids read a book.
The bicycles are green.
The dog is looking at the cat.
Telling parts
The telling part tells what someone or something does or is.
The boy caught a fish.
The balloons are flying in the sky.
Robbie has a large pizza.