because
1.The reason is和The reason why...is后面最好用that，不用because（很多用because的）。
2.because比for、as或since语气重。在答复why的时候必须用because，不可用for、as或since。
3.because of通常引导状语短语，很少引导形容词短语，如通常说He stayed at home because of the rain，很少说His stay at home was because of the rain。但在He stayed at home, this was because of the rain里的because of是正常的，这this并不指任何名词，却指stayed at home的意思。
4.Because he is ill is the reason why he cannot come里的Because不算正当，最好改作That。
