Simple? Compound? Complex?
A simple sentence consists of one independent clause (also called a "main clause").
A compound sentence has two or more independent clauses.
A complex sentence has one independent clause and one or more dependent clauses (also called "subordinate clauses").
A compound-complex sentence has two or more independent clauses and one or more dependent clauses.
Notice that the compound-complex sentence is simply a sentence that is both compound (2 or more independent clauses) and complex (1 or more dependent clauses). It is not always treated as a separate type.
We might visualize these definitions this way*:
An independent clause can be identified by the fact that it can be changed into a single yes/no question without deleting any words or adding any words except, possibly, the auxiliary verb do:
This is a simple web page. Is this a simple web page?Mary had a little lamb. Did Mary have a little lamb? (Note that the tense marker, in this case past tense, attaches to do, so that it appears we have changed had to have and inserted did. Actually, however, past tense was already here--we just moved it.)
If there are two independent clauses (i.e., if the sentence is compound), there will be two yes/no questions:
Mary had a little lamb, and it followed her to school. Did Mary have a little lamb? Did it follow her to school? (Sometimes the questions will make sense as a compound, joined by the same coordinating conjunction used in the original sentence: Did Mary have a little lamb, and did it follow her to school? Sometimes this won't work, or will be so awkward it seems ungrammatical.)
A dependent clause simply becomes part of the yes/no question formed from the independent clause:
Mary had a little lamb that followed her to school. Did Mary have a little lamb that followed her to school?
Notice that if we try to make that followed her to school into a yes/no question, we have to omit that and insert it to make the result grammatical:
That followed her to school *Did that follow her to school? (Did it follow her to school?)
A compound sentence must have either a coordinating conjunction or a semicolon (or possibly a dash or colon) between the independent clauses. Inserting a comma instead creates a comma splice, and using no joining at all creates a fused or run-on sentence.
The links below go to pages that provide both explanations of these terms and interactive exercises that let you practice applying the definitions. (Remember that compound-complex sentences may be seen as a combination rather than a fourth separate type.)
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