A sentence that opens with an introductory word, phrase, or clause requires a small pause between it and the rest of the sentence:

After the water began boiling I turned the flame down.   

Problem and cure: To head off misreading, a comma should be placed after the word "boiling."

In some cases, it isn’t necessary to place a comma after the introductory element. In the following, it is fairly clear how the prepositional phrase works without a comma present:

Before bowling the team met together to eat pizza.

But in other situations, the comma will mean the difference between clear communication and confusion:

In German nouns are capitalized.

A comma is essential in this sentence (after "German") to head off ambiguity.

common errors