連詞
連詞( conjunction) :用於引導從句以形成句子的一部分或修飾句子的構成要素，連詞是連線單詞,短語,從句或句子的一種虛詞。
從屬連詞
從屬連詞(subordinating conjunction)
從屬連詞這種連詞是用以引導名詞性從句和狀語從句的. 由從屬連詞所引導的句子叫從句,而含有從句的句子叫作複合句。
從屬連詞分類
大致可分為兩大類：
1、名詞性從句里的從屬連詞
that （無詞義，不做成分）
if，whether （表達是否的意義，但不做句子成分）
2、狀從里的從屬連詞，before，after，since，if，because，though等。
用法
在英語詞類中，連線詞可說是最容易掌握的一種。
從結構上說，英語連線詞分兩大類：並列連詞（coordinating conjunctions）和從屬連詞（subordinating conjunctions）。
並列連詞連線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.
從屬連詞連線兩個或兩個以上的分句，形成複雜句中的從屬分句。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.
從屬連詞用來連線各種從句。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。如動詞是持續性動詞，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.
舉例說明
上述連詞的用法，看起來不難，但錯誤也難免。下面是些好例子：
(1) This book may be used both as a text as well as a reference book.
這裡的“as well as”和“both”不能搭配，應該將“as well as ”改為“and”，使“both.......and......”變成關聯連詞（ correlative conjunction）
(2) He not only speaks Mandarin but also English.
這裡的“not only”應該移到“speaks”後面，使這個動詞兼顧兩個等立賓語：“Mandarin”和“English”。
(3) Although he is fat, but he is very weak.
英語裡的“although”和“but”是不見面的 ，因此這句里的“ but”要去掉，不然“although”就要去掉。
(4) Jim is not so strong like you.
這裡的“not so”必須和“as”連成一體；介詞“like”是不適宜的。
(5) No sooner had we reached our destination, they left.
"No sooner"必須和"than" 配成關聯連詞，把"than" 省掉是不對的，應該補上：
No sooner had we reached our destination than they left.
這句子的意思也可透過“as soon as”反映出來：
“As soon as we reached our destination, they left.”
(6) I took a taxi and which took me to the station.
這裡的並列連詞“and”是多餘的，必須去掉。如果要保留“and ”也可，但是形容詞分句里的關係代詞“Which”要改成主語“it” ，使整個句子變成並列分句如下：
I took a taxi and it took me to the station.
(7) My friend came up to me and held my hand, said good-bye excitedly.
這裡的“and”應該用來連線最後兩個謂語才對：
My friend came up to me, held my hand, and said good-bye excitedly.
不然，就要把“said”改成“saying”。
“saying”起主要作用。
