Structure Skill Toefl 6 : Adverb Clause Connectors

Sentences with adverb clauses have two basic patterns in English. Study the clauses and connectors in the following sentences:

He is tired because he has been working so hard.

Because he has been working so hard, he is tired.

In each of these examples, there are two clauses: he is tired and he has been working so hard. The clause he has been working so hard is an adverb clause that is introduced with the connector because. In the first example, the connector because comes in the middle of the sentence, and no comma (,) is used. In the second example, the connector because comes at the beginning of the sentence. In this pattern, when the connector comes at the beginning of the sentence, a comma (,) is required in the middle of the sentence.

Example:

_____ arrived at the library, he started to work immediately.

(A) The student
(B) When
(C) He
(D) After the student

In this example, you should recognize easily that the verb arrived needs a subject. There is also another clause, he started to work immediately. If you choose answer (A) or answer (C), you will have a subject for the verb arrived, but you will not have a connector to join the two clauses. Because you need a connector to join two clauses, answers (A) and (C) are incorrect. Answer (B) is incorrect because there is no subject for the verb arrived. Answer (D) is the best answer because there is a subject, student, for the verb, arrived, and there is a connector, after, to join the two clauses.

The following chart lists common adverb connectors and the sentence patterns used with them.

Source : http://www.toeflskill.com/2011/02/skill-5-adverb-clause-connectors.html