When an adverb clause -- it has a subject and verb and a word out in front of it that is part of the clause and is used to answer when, where, why, how -- starts a sentence, it is always followed by a comma.
...If you see her later, please ask her to...
...When she drove away from the scene, did you notice that...
...Because it was raining hard, we made a turn into...
Sometimes these clauses are "elliptical," which means the subject and/or the verb are missing, they still have a comma after them.
...If necessary, we will send it on...
"If necessary" is short for "if it is necessary."
...As expected, they were all...
"As expected" is short for "as it was expected."
Elliptical clauses are punctuated exactly the way the full clause is punctuated.
On April 6 and 7, I will be giving a four-hour seminar on sentence structure and clauses. Details to come. This is possibly the most important aspect of punctuation to understand!
Happy punctuating!
Margie