A simple sentence consists of only one clause.
A compound sentence consists of two or more independent clauses.
A complex sentence has at least one independent clause plus at least one dependent clause.
A set of words with no independent clause may be an incomplete sentence, also called a sentence fragment.
A sentence consisting of at least one dependent clause and at least two independent clauses may be called a complex-compound sentence or compound-complex sentence.
- I like trains.
- I don't know how to bake, so I buy my bread already made.
- I enjoyed the apple pie that you bought for me.
- The dog lived in the garden, but the cat, who was smarter, lived inside the house.
- What an idiot.
Sentence 1 is an example of a simple sentence.
Sentence 2 is compound because "so" is considereda coordinating conjunction in English, and sentence 3 is complex.
Sentence 4 is compound-complex (also known as complex-compound). Example 5 is a sentence fragment.
The simple sentence in example 1 contains one clause.
Example 2 has two clauses (I don't know how to bake and I buy my bread already made), combined into a single sentence with the coordinating conjunction so.
In example 3, I enjoyed the apple pie is an independent clause, and that you bought for me is a dependent clause; the sentence is thus complex.
In sentence 4, The dog lived in the garden and the cat lived inside the house are both independent clauses; who was smarter is a dependent clause.
Example 5 features a noun phrase but no verb. It is not a grammatically complete clause.