从属连词是连词的一种，用来引导名词性从句或状语从句。连词通常引导一个从句，修饰主句。 由从属连词所引导的句子叫从句,而含有从句的句子叫作复合句
·从属连词可分为三大类：
1、that （无词义，不做成分）
if，whether （表达是否的意义，但不做句子成分）
2、连接代词：who，whom，whose，what，which，whatever, whoever, whichever（有词义，在句子中可作主语、表语、宾语）
3、连接副词：when，where，why，how，how many，how long，how far，however，whenever，wherever （有词义，作从句的状语）
用法：
从结构上说，英语连接词分两大类：并列连词和从属连词。
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。
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.
·
