错误：He did not come to school. Because he was ill.
正确：He did not come to school because he was ill.
这是一个非常容易犯的错误。原则上来说，because从句是不可以没有主句单独使用的。上例中将because从句另起一句，与前面的主句分开，是不正确的。
除非是在特殊的情境下，上下文内容很清晰，即使省略主句也不影响读者的理解时，可能会出现省略主句的情况，例：I don't think I'll go to the party after all. Actually ... Because I'm feeling a bit tired.
在日常口语中经常出现省略的情况，如：
A: Why didn't you come to the party?
B: Because I was sick.
这里完整说法其实是：I didn't come to the party because I was sick.
