Commas: Introductory Elements

Commas should be used to separate introductory elements from the main part of the sentence. Introductory elements can be single words (i.e. interjections), phrases (i.e. prepositional phrases), or clauses (i.e. adverb clauses).
Example: Wow, you really know your stuff!
On Tuesday, we will have ice cream.
Before we leave for the circus, I need to find my clown nose.