Comma After Introductory Clauses
Introductory clauses are dependent clauses which are found at the beginning of the sentence (although they can be moved to the end of the sentence, too, without confusing the meaning of the sentence). After a dependent introductory clause, we use a comma to separate the introductory clause from the independent clause. The dependent introductory clause may start with an adverb or conjunction like although, if, or when.
As the man was walking into the store, he came face-to-face with his childhood sweetheart.
Because the rain was torrential, the day’s Little League games were postponed.
Grabbing her umbrella, Kate raced out of the house.
Now then, we should head back home because the baby needs a nap.