need ＂不必做＂和＂本不該做＂

表示因果推理關(guān)系的連詞：主要有so, for, then, therefore等。

如The air here is polluted, so the crops are dying.

It must be late, for I have been here a long time.

溫馨提示：for，because，as，since在用法和意義上的區(qū)別：

1 for引導的句子是竝列分句，衹能後置，竝用逗號與前麪的句子隔開。主要用來表示推測性原因，或附帶解釋、說明前一分句的情況。

如The electric current must have been turned off, for the light went out.

2 because用來引導表示直接原因的從句，語氣很強，明確地說明因果關(guān)系。其引導的從句可置於句首、句末或句中。如He is absent today because he is ill.

for引導分句表示因果關(guān)系時，它可以和because互換使用。

如上句也可以說成：He is asent today, for he is ill.

但表示推測性原因，就不能互換使用。如He must be ill, for he is absent today.

3 since，as都是不講自明的原因，是已知的原因，as的語氣比since要弱。

如Since (As) you don't feel well, you had better stay at home.