從結構上說，英語連接詞分兩大類：

并列連詞連接兩個或兩個以上地位平等的字、詞組或分句。例如：

(1) Air and water are indispensable to me.

(2) She likes going out with friends or playing outdoor games.

(3) Most workers have a good income, so they look very happy.

從屬連詞連接兩個或兩個以上的分句，形成復雜句中的從屬分句。例如：

(4) He said that he did not want to go .

(5) Many things have happened since I last saw him.

(6) You may come if you want to.

從屬連詞用來連接各種從句。

until(till)直到，在用until表達時間狀語的句子中，主句中的動詞是要十分小心去選擇的。

如動詞是持續(xù)行動詞，它要用肯定句，如：

I studied hard until 12 o'clock last night.

如果動詞是瞬間截止性動詞，則要用否定句，如:

He didn't go to bed until his mother came back.

由since,for,by,before, 來引導的時間狀語從句。

since 引導的時間狀語是動作的開始時間，如:

I have studied English since 1990.

而by引導的時間狀語通常是動作的結束時間，如：

I had learned 25 English songs by the end of last term.

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