What type of sentence consists of just one independent clause?

A sentence consisting of just one independent clause is called a simple sentence.

An independent clause contains a subject and a predicate and can stand on its own as a complete thought. “The children laughed” is a simple sentence because it has one independent clause. A simple sentence can still include a compound subject, a compound verb, adjectives, adverbs, or prepositional phrases: “The young children laughed and waved at the parade” remains simple because it has only one clause.

The key issue is clause structure, not length. A short sentence can be complex if it contains a dependent clause, as in “Although it rained, we played.” A long sentence can still be simple if its added words are phrases rather than additional clauses.

The other standard types are compound, complex, and compound-complex sentences. A compound sentence joins at least two independent clauses, usually with a coordinating conjunction. A complex sentence combines one independent clause with at least one dependent clause, while a compound-complex sentence contains at least two independent clauses plus a dependent clause. Writers use all four structures to vary rhythm and emphasis.