A clause is a group of words which contains a subject and a predicate or verb. It can be as short as one word (EXAMPLE: Wait!) or a group of words that contains a subject and a predicate, and usually forms only part of a sentence. Each sentence consists of one or more clauses.
EXAMPLE: He could not lift the box because it was too heavy.
In the example, the first clause He could not lift the box can stand alone by itself and is clear in its meaning. It is known as the Main Clause or Independent Clause. The main clause, like all main clauses, has a subject and a verb, and does not depend on the other clause to make a complete sentence.
The second clause because it was too heavy is incomplete in its meaning and is called a Dependent Clause or Subordinate Clause. It too has a subject and verb but it depends on the first clause preceding it to make a complete sentence. A dependent clause may come before a Main clause as follow:
EXAMPLE: I respect him for his honesty and sincerity though I dislike him.
EXAMPLE: Though I dislike him, I respect him for his honesty and sincerity.
In the two examples, the Dependent Clause is in bold. The second example shows the dependent clause preceding the Main Clause. When the dependent clause comes before the main clause, a comma has to be used at the end of the dependent clause.
A clause may also be inserted in the middle of another clause.
EXAMPLE: Orchids, which have air roots, are tropical flowers
The Main Clause here is "Orchids are tropical flowers."
The Dependent Clause: "which have air roots" occupies the middle position.
A sentence can have more than one main clause.