We know that a simple sentence is a complete thought with one person or thing doing one action.
Bob rode his bike.
Bob is the subject. The action is rode.
Simple sentences always express only one main thought, but that main thought may have a compound subject or a compound verb.
When a subject is compound, two or more people or things are doing an action.
Bob and Mike rode their bikes.
This simple sentence has one main idea, but it has a compound subject. There are two boys doing the action.
Here are some more sentences with a compound subject:
Sue and her sister are making dinner.
Mom and Dad both worked today.
The boys and the girls in second grade will compete in a four-square tournament.
Each of these sentences is a simple sentence – but each one has a compound subject.
A simple sentence can also have a compound verb.
The subject can do two actions, yet the sentence is still one complete thought.
Grandma made sandwiches and poured some lemonade.
This sentence has one complete thought; it tells what Grandma did. She made sandwiches and poured lemonade.
Here are some other simple sentences with a compound verb – two actions with one complete thought:
The tide rises and falls twice every day.
Fluffy ran to the door and barked at the intruder.
Janice read the book and studied for her test.
A simple sentence has a subject and a verb that expresses one complete thought. A simple sentence may have a compound subject or a compound verb. Be sure to end every simple sentence with a period.