A sentence is the largest grammatical unit in any language. In English, a sentence always contains a subject and a predicate.

A sentence must contain at least one main clause and it may also contain additional main clauses or subordinate clauses. In writing, a sentence begins with a capital letter and ends with a full stop, interrogation mark or an exclamation mark.

The Vikings reached North America in the eleventh century.
Why did you break my window?
What a beautiful flower!

Note that an imperative sentence doesn’t always contain a subject.
Stop!
Get lost!

Clause
A clause is a grammatical unit consisting of a subject and a predicate. In the following sentences the groups of words given in bold text are examples of clauses.

Those whom the God loves die young.
He told me that he would come.
Why he has behaved in this manner is not known to me.
Will you please tell me where you found this book?
Where there is a will there is a way.
There was light on all sides.

There are two principal types of clauses: main clauses and subordinate clauses.

A main clause can stand alone to make a complete sentence by itself. A subordinate clause, on the other hand, can’t stand alone: it must be attached to another clause within a larger sentence. Every sentence has at least one main clause. Some sentences may contain two or more main clauses and one or more subordinate clauses. For example, the sentence ‘They visited us’ consists of a single main clause, while the sentence ‘Mike cooked dinner and Susie tidied the room’ consists of two main clauses connected by and, and the sentence ‘He told me that he had finished his work’ consists of a main clause and a subordinate clause beginning with that.