A simple sentence is a sentence with only one complete thought. It usually has only one subject and one verb.
Bob drives a truck.
My cat ran away.
I like to eat a hotdog at the ballpark.
In each of these sentences, one subject is doing one thing.
Sometimes a simple sentence has a compound subject. Two subjects are doing one thing.
John and Mary went to the movies.
The boys and the girls are having a tug of war.
Cars and trucks use the highway.
Even though each sentence has two subjects, the two subjects are doing one thing, so it is a simple sentence.
A simple sentence can also have a compound verb. One subject is doing two things.
Jimmy plays basketball and runs track.
Martha sang a song and played the piano.
Jack called my name and pounded on my door.
In each of these simple sentences, the subject is doing two things.
A simple sentence is usually one subject doing one thing. However, a simple sentence can also have two subjects doing one thing or one subject doing two things. It is still a simple sentence because there is only one complete thought.
▶Now you try it. Which of these two sentences is a simple sentence? Bob ate an ice cream cone while his dog watched. Bob and his dog went to the ballpark. Did you pick the second sentence? Good for you!