从结构上说，英语连接词分两大类：

并列连词连接两个或两个以上地位平等的字、词组或分句。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.

从属连词连接两个或两个以上的分句，形成复杂句中的从属分句。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.

从属连词用来连接各种从句。

until(till)直到，在用until表达时间状语的句子中，主句中的动词是要十分小心去选择的。

如动词是持续行动词，它要用肯定句，如：

I studied hard until 12 o'clock last night.

如果动词是瞬间截止性动词，则要用否定句，如:

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

由since,for,by,before, 来引导的时间状语从句。

since 引导的时间状语是动作的开始时间，如:

I have studied English since 1990.

而by引导的时间状语通常是动作的结束时间，如：

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

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