赖世雄教你学英语语法
两句的连接方法
3-4:副词从句的位置
because引导的副词从句若置于主句之后，because之前就不必加逗点。但because从句若置于主句之前，则与主句之间就置逗点。由此我们得知一个惯用的法则：任何一个连接副词所引导的从句置于主句之后，两句间通常不置逗点。但若该副词从句置于主句之前，则两句间通常置逗点。
例：though he did poorly on the exam,his father didn't blame him.
=his father didn't blame him though he did poorly on the exam.
（虽然他考试考得不好，他父亲并未责备他。）
if i am free,i'll write you.
=i'll write you if i am free.
（我有空的话就会写信给你。）
as soon as he heard the story,he cried.
=he cried as soon as he heard the story.
（他一听到这个消息就哭了。）
不过告诉你的只是个惯用的法则，偶尔也会见到有人将副词从句置于主句之后时，仍于两句间置逗点。如：
例：i like him,because he is nice.
这种用法不能算是常态用法，懂英语而且写作严谨的人都会避免这样的用法。