Place a comma after an introductory adverb clause.
An adverb clause shows time, place, degree, extent, cause, or condition. It is a subordinate clause which begins with a subordinating conjunction.
Correct: Before the curtain fell, the actors bowed.
Correct: If the next two nights are sellouts, the play will be extended.
Note that if a sentence ends with an adverb clause, no comma is used. The subordinating conjunction is enough of a separation.
Incorrect: The play's run will be extended, if the next two nights are sellouts.
(No comma needed with adverb clause at end of sentence.)
Correct: The play's run will be extended if the next two nights are sellouts.