Writers, especially beginning writers, are often cautioned against using passive voice in their writing because its use slows down the pace.

Another construction that can make your writing plod is the dummy subject.

When we use the words it and there to begin a sentence without a referent (a noun the pronoun is referring to), we’re using a dummy subject.

关于这个问题，可参考：

http://www.ucl.ac.uk/internet-grammar/minor/dummy.htm

http://www.dailywritingtips.com/the-dummy-subject/

http://grammar.about.com/od/d/g/Dummy-It.htm