As the name implies, an introductory clause is a dependent clause at the beginning of a sentence. The preceding sentence, in fact, contained such a clause. Use commas to separate introductory clauses and certain phrases from independent clauses. Introductory phrases of more than five words or phrases containing verbal elements also require commas. Dependent clause openers include:
Some examples are:
"As expected, she could not turn in the homework on time."
"In the fall of last year, we held a family reunion." (use a comma after a phrase containing five or more words)
"To decide, they held a contest." (use a comma after an introductory phrase, regardless of length)
This is one area where comma use can be tricky. When in doubt, stop and think about what you are trying to say. Does the phrase warrant a pause? If so, a comma is probably needed.