Use commas to set off transitional words and phrases, introductory clauses, or introductory phrases to signal where the introductory element finishes and the main part starts.
Place a comma after a transitional word or phrase that begins a sentence.
Moreover, the opening of an export market would help expand the market for key escrow encryption.
In addition, several companies and individuals have proposed
commercial key escrow approaches.
--Dorothy E. Denning, "The Case for Clipper," Technology Review
Place a comma after an introductory dependent clause.
Although key escrow is voluntary, critics say that the introduction of Clipper points national policy in a disturbing direction.
Normally, use a comma after an introductory prepositional or verbal phrase. However, you may omit the comma after a short introductory phrase if no ambiguity is possible.
For the first time, researchers have used DNA analysis to identify the animal tissue in 4,000-year-old rock paintings.
--"Science and the Citizen," Scientific American
Despite the error the experiment was successful.
Combining surface area with depth, we calculated the volume of the pond.
Do not place a comma after an introductory participial or gerund phrase if the phrase forms part of the subject or verb of the sentence.
Combining surface area with depth was our principal mthod for calculating the volume of the pond.