A simple sentence is made up of one and only one independent
clause. It has a complete subject and a complete predicate, and nothing more.
Consider the following example:
Jerry ran away.
This sentence has only three words in it: a simple subject,
a simple predicate, and an adverb. It is one of the most simple sentences in
our language.
Simple sentences, however, can also be quite long. For
example:
Martha and Noah sang, danced, and
performed magic tricks at the fundraiser event last Tuesday evening.
Some students get confused when they see simple sentences
with compound subjects and/or predicates. The above example has a compound
subject (Martha and Noah) and a compound predicate (sang, danced, and
performed). Just because a sentence has a compound subject and/or a compound predicate
does not mean that the sentence itself is compound. In this case, the
sentence is still a simple sentence, just with a compound subject and
predicate.