表示因果推理關系的連詞：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引導的句子是并列分句，只能后置，并用逗號與前面的句子隔開。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，明確地說明因果關系。其引導的從句可置于句首、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.