请教这个从句前面为什么有个逗号？

来源：百度知道 编辑：UC知道 时间：2024/06/01 01:42:47
While she was having a cup of tea in the living room,someone knocked at the door.
如果从句放在后面，不需要用逗号隔开。那么someone knocked at the door是从句，为什么前面有个逗号呢？

someone knocked at the door不是从句，是主句
she was having a cup of tea in the living room是由while引导的从句

someone knocked at the door不是从句，是主句
she was having a cup of tea in the living room是由while引导的从句

楼上说得对。

再加一句，

放在后面，有没有逗号都可以的。

不过，用了逗号，就不能用that ，只能用which.