The main clause gives you the main idea of the sentence, and it makes sense without any other information. It can stand alone as a sentence. Example:
When their father went to work, the boys played all day long in the tree. (“The boys played all day long in the tree” is the main clause. It can stand alone as a sentence.
Another name for the main clause is the independent clause.
The subordinate clause depends on the main clause for its meaning. It cannot stand alone as a sentence. In the sentence above, “When their father went to work,” is the subordinate clause. It is subordinate because you need something to come after “When their father went to work” – it is not a complete sentence on its own.
Another name for the subordinate clause is the dependent clause.
The main clause is often separated from the subordinate clause by a comma.
More examples of main clauses:
A good man lived in the village, where many people were poor.
Because he grew vegetables, he gave everybody food.
More examples of subordinate clauses:
If you drink dirty water, you will get sick.
She had just sat down, when the doorbell rang .