A simple sentence is a sentence that is made of a single independent clause and no dependent clauses. It contains a subject and predicate and nothing else—for example:
The cat crouched.
The rain is falling.
The mail hasn’t come.
Each of these sentences contains a single clause composed of a subject (The cat, The rain, The mail) and a predicate (crouched, is falling, hasn’t come).
Simple sentences can get quite long without becoming compound or complex sentences—for example:
A few years ago, the man now protesting the government was on unemployment and had no health insurance.
Although this sentence is relatively long, it still contains nothing more than a subject (the man now protesting the government), a predicate (was on unemployment and had no health insurance), and a non-clausal adverbial phrase (A few years ago,) modifying the verb was. Another example:
In an hour, I’m going home.
Because the prepositional phrase In an hour is not a clause, this is a simple sentence.
