Study the following sentences.
They started in the morning. (Started when? – in the morning)
They started before the sun rose. (Started when? – before the sun rose)
It is clear that in sentence 1 and sentence 2 the group of words in italics do the work of an adverb as they modify the verb started, showing when the action was performed.
Here the group of words ‘in the morning’ is an adverb phrase. It does not have a subject or predicate of its own. It makes sense, but not complete sense.
Now consider the group of words ‘before the sun rose’. It has a subject and a predicate of its own. Still, it is not a complete sentence. In fact, it is part of a large sentence.
Such a group of words which forms part of a sentence, and has a subject and a predicate of its own is called a clause. Since the clause ‘before the sun rose’ does the work of an adverb it is called an adverb clause.
More examples are given below.
Start when you are ready. (Adverb clause – when you are ready)
Will you wait till I return? (Adverb clause – till I return)
If you make a promise, you must keep it. (Adverb clause – if you make a promise)
I shall remain where I am. (Adverb clause – where I am)
Just as he entered the room the clock struck twelve. (Adverb clause – just as he entered the room)
He finished first though he started late. (Adverb clause – though he started late)