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need ＂不必做＂和＂本不该做＂
表示因果推理关系的连词：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.
温馨提示：for，because，as，since在用法和意义上的区别：
1 for引导的句子是并列分句，只能后置，并用逗号与前面的句子隔开。主要用来表示推测性原因，或附带解释、说明前一分句的情况。
如The electric current must have been turned off, for the light went out.
2 because用来引导表示直接原因的从句，语气很强，明确地说明因果关系。其引导的从句可置于句首、句末或句中。如He is absent today because he is ill.
for引导分句表示因果关系时，它可以和because互换使用。
如上句也可以说成：He is asent today, for he is ill.
但表示推测性原因，就不能互换使用。如He must be ill, for he is absent today.
3 since，as都是不讲自明的原因，是已知的原因，as的语气比since要弱。
如Since (As) you don't feel well, you had better stay at home.
