The Complex Sentence
1. James rejoiced when he reached home, because he saw his mother.
2. John, who has worked hard, is to be rewarded.
3. Though the road be rough and the distance great, he will arrive early, as he promised.
In the first sentence, James rejoiced expresses the principal thought and is modified by the clause when he reached home, which tells the time he rejoiced, and by the clause because he saw his mother, which tells the cause, or why he rejoiced.
In the second sentence, John is to be rewarded expresses the principal thought; John is modified by the adjective clause who has worked hard, which describes John as the actor, - the one who worked hard.
In the third sentence he will arrive early expresses the principal thought, and is modified by the clause as he promised, which compares the time he will arrive with the time he promised.
Will arrive is modified also by the clauses though the road be rough and the distance be great, which both concede or suppose that he will not arrive under such difficult circumstances.