A group of words consisting of a subject and a predicate is called a clause.
The dog barks.
The sun shines.
Ann sang a song.
There are two principal types of clause: a main clause can stand alone to make a sentence by itself, while a subordinate clause must be attached to another clause within a larger sentence.
Every sentence must contain at least one main clause, though a sentence may contain two or more main clauses, and in any case may additionally contain one or more subordinate clauses.
For example, the sentence Alice wrote the letters consists of a single main clause, while the sentence Alice wrote the letters and Peter posted them consists of two main clauses connected by and, and the sentence Peter started making dinner while Alice tidied the lounge consists of a main clause plus a subordinate clause beginning with while.
A simple sentence consists of a single main clause.
Mike started making dinner.
Susie tidied the lounge.
A compound sentence consists of two or more main clauses. The clauses are often connected by a conjunction like and, or, but or yet.
Mike smokes but Peter doesn’t.
Alice wrote the letters and Peter posted them.
A complex sentence consists of one main clause plus one or more subordinate clauses.
Alice said (main clause) that she would come (subordinate clause).
You may sit (main clause) wherever you like (subordinate clause).
Will you wait (main clause) till I return (subordinate clause)?
If you eat too much (subordinate clause) you will fall ill (main clause).
A compound-complex sentence consists of two or more main clauses plus one or more subordinate clauses.
After she left university (subordinate), Alice moved to London (main) and her boyfriend followed her (main).