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I am no native speaker and always confused about the comma in introductory phrases, in particular in prepositional phrases. Is there any hard rule when a comma must be set? If I make a google search for certain phrases, I often find both variants. Typical examples where I am not sure whether a comma must be set are:

  • In this case[,] we must...
  • For simple problems[,] the algorithm...
  • From Lemma 1.2[,] we obtain...
  • For a typical user[,] the algorithm...
  • In our theory[,] we...
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2 Answers

up vote 6 down vote accepted

Larry Trask’s advice in cases like this is to see what happens if you remove from the sentence the words marked off by the comma. If you are left with a meaningful sentence, then the comma is appropriate. If no meaningful sentence remains, you don’t need the comma.

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Trask is primarily talking about paired "bracketting commas" delineating non-essential clauses. Near the end of that link he covers cases where the "supplementary clause" appears at the beginning or end of a sentence (with a notional comma assumed before or after the clause). The fact that you can still have a meaningful sentence without the clause doesn't mean you must always enclose it in commas - just that you can. –  FumbleFingers Dec 18 '11 at 16:17
    
@FumbleFingers: Quite so. Knowing the dangers of selective quotation, I hoped Boris would go to the source. –  Barrie England Dec 18 '11 at 16:26
    
I think there's a tendency to use the comma, notwithstanding RiMMER's position. I personally would change your comma is appropriate to comma may well be appropriate, but really I think it's largely a matter of style in OP's specific context. –  FumbleFingers Dec 18 '11 at 16:34
    
@FumbleFingers: I used to be a minimalist in punctuation, but am swinging back slightly in the opposite direction. –  Barrie England Dec 18 '11 at 16:35
    
Oh dear - does that make me a dinosaur? I still think providing readability doesn't suffer, less is more in punctuation as in so many things. –  FumbleFingers Dec 18 '11 at 16:37
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Comma sense—a fun-damental guide to punctuation suggest to use the comma to set off introductory elements, which are reported to be:

  • an adverb: First, I need to call my girlfriend.
  • a prepositional phrase: After dinner, let's go to see a movie.
  • an appositive: A stumbling giggler, Lumpy was hardly prepared for the relay baton suddenly being thrust upon him.
  • a participial phrase
  • an infinitive phrase: To be honest, I think you are over-reacting.
  • a dependent clause: If you are going to leave this room, I am not going to call you anymore.

This is an example used by that guide to show what happens if you don't use the comma after an introductory element:

After retiring my wife, my parents, the kids, and I plan to travel around the country.

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+1 for the great example. –  Peter Shor Dec 18 '11 at 18:53
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