单独的状态型副词，或者副词型短语，放在句首一般都要加逗号。例如
Fortunately,we got the station just before the the bus was about to start.
Three days ago, a sandstorm occurred in the northweat of that dry area.
但是，这并不是一成不变的。有时，有些也可以不加逗号。例如
Nowadays more and more people begin to realize the importance of enviornmently protection.
一般不用。
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