An independent clause is a group of words with a subject doing something. In an independent clause, only one thing is happening. A simple sentence is one independent clause. A compound sentence is two independent clauses joined together. We can also say that a compound sentence is two simple sentences joined together.
Sometimes a simple sentence shows action.
Bob is riding his bike to school.
In this sentence, Bob is doing an action. He is riding.
Sometimes a simple sentence shows existence with no real action.
Mary is a librarian.
In this sentence, Mary isn’t doing any action; she just is. She exists.
An independent clause (which is also a simple sentence) needs a subject and a verb. Look at this incomplete sentence: The big yellow bus down by the corner. This sentence has a subject, but there is no action and no existence. We need to add a verb to this incomplete sentence, like this:
The big yellow bus down by the corner is blowing its horn.
Here is another incomplete sentence. This one is missing a subject: very fast down the road.
We need to put a subject into this sentence to tell what or who is going very fast down the road.
Here is a complete simple sentence: That red car is going very fast down the road.
Understand that a simple sentence is one independent clause. An independent clause needs a subject and a verb. A simple sentence needs the same thing.
▶It’s your turn. Make this incomplete sentence into a good simple sentence by adding whichever it needs – either a subject or a verb. The new fifth grade teacher. Did you figure out that you need to add a verb to make this sentence complete? Good for you!