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Here is an example:

Traffic is terrible driving to downtown! That is, if you arrive from the south. Traffic from the north isn't that bad.

In the above sentence, the comma after "that is" feels grammatically unnecessary. At the same time, the sentence feels like it would would flow way differently than if it were just "That is if you arrive form the south."

Another example:

I don't trust them to babysit my kids. You know, since they had that incident last year.

I think we can all agree that it would have been much better if the sentences were just one complete thought, like "I don't trust them to babysit my kids since they had that incident last year." But I feel like there should be a grammatically correct way to express somebody who makes a complete thought, ends it, and then regrets not adding that additional clause.

My main issue is that these are dependent clauses ("if", "since") that are not normally preceded by a comma, yet it feels so weird to NOT have the comma in these cases.

At the end of the day, what the comma is trying to accomplish is the pause that would normally occur when these types of sentences are spoken. In that case, maybe an ellipsis would be more appropriate? It feels like it works more in some cases than others, depending on how long you want the pause to feel.

Traffic is terrible driving to downtown! That is... if you arrive from the south. Traffic from the north isn't that bad.

doesn't seem right.

I don't trust them to babysit my kids. You know... since they had that incident last year.

could work?

closed as primarily opinion-based by anongoodnurse, jimm101, Helmar, tchrist Nov 9 '16 at 14:47

Many good questions generate some degree of opinion based on expert experience, but answers to this question will tend to be almost entirely based on opinions, rather than facts, references, or specific expertise. If this question can be reworded to fit the rules in the help center, please edit the question.

  • Comma with "that is", no comma with "you know", which I take to mean "remember that". – aparente001 Nov 7 '16 at 9:16
  • This may well be closed as there is no definitive answer. I'd use either 'That is, if ...' or 'That's if ...' with the former, and either a dash (fair pause) or ellipsis (longer pause) with the second example. I agree in general about the pause ... weight of punctuation relation. – Edwin Ashworth Nov 9 '16 at 0:19
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Consider the sentence

You know, since they had that incident last year.

The part of the sentence starting with "since" is not really a dependent clause of the the phrase "you know," is it? If it were, the sentence would be saying that the reason the subject ("you") knows something is because "they" had an incident.

As you observed, the phrase after the comma in each example would fit quite well if it had been inserted at the end of the previous sentence. The words "that is" or "you know" are, in effect, phrases that have been inserted parenthetically between two clauses. A parenthetical phrase like this is typically set off with commas before and after the phrase. In these examples, the comma before the parenthetical phrase has been superseded by punctuation that formally ends a sentence. But in each example we still have a parenthetical phrase and I think it still makes sense to set it off with a comma at the end.

Setting such a phrase off with an ellipsis would suggest to me a longer pause at that point in the sentence than one normally uses with such a construction.

  • Thinking of that phrase as a parenthetical makes it much more clear, thank you for the insight. – JHixson Nov 7 '16 at 7:35

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