What is proper sentence structure?

For a sentence to have proper structure, it has to contain at least one independent clause, or a group of words with a subject and a predicate that expresses a complete thought. A sentence comprised of only one independent clause is a simple sentence.

There are four kinds of sentence structures: the simple sentence, with only one independent clause; the complex sentence, with two independent clauses joined by a semicolon or a coordinating conjunction; the complex sentence, with a subordinate clause and an independent clause; and the compound-complex sentence, with at least two independent clauses and at least one subordinate clause.