1.起承上启下的作用
Otherwise，we should commit a serious mistake.否则，我们会犯严重错误。
That is to say，we have to study harder.也就是说，我们应该更加努力学习。
2.起解释作用
He is right，I think.我认为他是对的。
The trees in that area，it is said，are mostly over thirty feet tall.
那一带的树据说多数都三十多英尺高。
3.对整句进行解释
Strangely，he has not been here lately.奇怪的是他最近一直没有来。
By the way，where is Xiao Li？ 顺便问一问，小李在哪儿？