A simple sentence has only one independent clause, or complete thought. Thus, the simple sentence, like all complete sentences, should contain a subject and a predicate. Although a simple sentence can contain compound elements such as a compound predicate and/or compound subject [Joe and Jane like to eat spaghetti and meatballs], it is only one clause.
Simple sentence – Joe likes to eat spaghetti.
[subject – Joe / predicate – likes to eat spaghetti.]
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A compound sentence consists of two or more simple sentences (two independent clauses). Because two complete sentences are being combined, a comma along with a coordinating conjunction (ex. – and, or, but) or, under certain conditions, a semicolon is used to combine the independent clauses.
Compound sentence – Joe likes to eat spaghetti, but Mary likes to eat hamburgers.
[Notice that the sentences are joined with the conjunction but as well as a comma.]
Compound sentence – Joe likes to eat spaghetti; Mary likes to eat hamburgers.
[Here, the sentences are joined with a semicolon only. Semicolons can be used effectively when the two sentences are closely related. In this compound sentence both sentences discuss eating preferences.]
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A complex sentence consists of one independent clause and one dependent clause. So, one clause in the sentence should be a complete thought while the other clause cannot exist as a sentence by itself. One way to identify the dependent clause is to look for the subordinating conjunction (ex. – while, because, since, when, while, after, before) or the relative pronoun (ex. - who, that, which) that precedes it.
Complex sentence – Because Joe’s mother never cooked spaghetti when he was young, Joe likes to eat spaghetti.
[In this example, the first clause and second clause in the sentence are dependent since they both contain a subject and verb and begin with a subordinate conjunction (because and when), and the third clause beginning after the comma is an independent clause.]
Complex sentence – She likes to eat at the diner that serves free bagels on Sundays.
[Here, the relative pronoun that begins the second clause and causes it to be dependent.]
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A compound-complex sentence combines a compound and a complex sentence. A compound-complex sentence consists of at least two independent clauses and at least one dependent clause.
Compound-complex sentence – Because Joe’s mother never cooked spaghetti when he was young, Joe likes to eat it often, and Mary enjoys eating hamburgers because she used to eat them with her late father.
[This sentence has two independent clauses: Joe likes to eat it often and Mary enjoys eating hamburgers. In this case, it has three dependent clauses as well (the first begins with because; the second begins with when, and the final dependent clause begins with because as well).]
Remember that it is best to use a variety of sentence types when you write. If one type is overused, the writing will begin to sound repetitive and monotonous.