You should put a comma after an introductory clause or phrase:
Though the agency had studied this issue before, it went ahead with another study. (Introductory dependent clause.)
If I were you, I would research the case thoroughly. (Introductory dependent clause.)
After researching the issue, the committee settled the dispute before the media even noticed. (Introductory elliptical clause.)
In determining the application of this rule, the committee will balance the competing factors. (Introductory prepositional or present-participial phrase.)
Exhausted by the long campaign, the candidate took an extended vacation. (Introductory past-participial phrase.)
Because of the sensitive nature of the controversy, the Senate Judiciary Committee held an executive session. (Introductory prepositional phrase.)
Exception to the Rule
There is, of course, an exception. You may omit the comma following a short introductory phrase:
On Thursday the committee decided the dispute. In 1954 the Supreme Court desegregated the public schools.
But if you omit this comma once, you should do the same throughout the paper for all short introductory phrases.
Previous: Commas and Independent Clauses Next: Adverbial Phrases Between Subject and Verb