A clause is a group of words which may be used as a whole sentence, or as part of a sentence. A clause must contain a subject and a verb.
Example: Milo rode his bicycle down the hill.
This sentence contains a subject and a verb. This is a one-clause sentence.
Example: Milo fell of his bicycle| when he rode his bicycle down the hill.
This sentence contains two subjects and two verbs. This sentence has two clauses. However, there is a difference between clause 1 and clause 2.
The group of words in clause 1 [Milo fell of his bicycle] makes complete sense and can stand alone. This clause is referred to as the main clause or independent clause.
On the other hand, the group of words in clause 2 [when he rode his bicycle down the hill] don’t express a complete thought. Therefore, they are meaningless independently. They make sense only when added to the main clause. They are called subordinate clause.