A complex sentence contains one independent clause and at least one dependent clause.

A dependent clause is a group of words that contains a subject and verb but does not express a complete thought. A dependent clause cannot be a sentence. Words that introduce dependent clauses are after, although, as, as if, because, before, even if, even though, if, in order to, since, though, unless, until, whatever, when, whenever, whether, and while.

Use a comma after an introductory dependent clause, regardless of the length of the clause.

Examples
When the data on the screen indicates a problem with the network, contact your LAN administrator.
If you add the Intuity system to a switch, you must also administer the switch.

If the dependent clause comes at the end of a sentence, do not use a comma.

Examples
Contact your LAN administrator when the data on the screen indicates a problem with the network.
You must also administer the switch if you add the Intuity system to a switch.

However, use a comma near the end of a sentence to separate contrasted coordinate elements or to indicate a distinct pause or shift.

Examples
Refer to the drive for a compact disc as the CD-ROM drive, not the CD-ROM player or the CD drive.
Write voicemail as one word, whether as a noun or an adjective.