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A professor once told me never to start a sentence with "That is." Was he right about that? Does "That is" in the beginning preclude the sentence from being an independent clause? I don't know anymore, because I've been seeing it lately either at the beginning of a sentence or after a semi-colon. Please have a look at the slight variation of this sentence:

It was the right thing to do, that is, it was the best option for everyone.

It was the right thing to do; that is, it was the best option for everyone.

Is that first one a comma splice, or is it correct? Is the comma, in fact, separating an independent clause from a dependent one?

Thanks.
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SnarfIs that first one a comma splice
Yes.
SnarfIs the comma, in fact, separating an independent clause from a dependent one?
No.
Oh, so he was wrong! Interesting. I see it both ways so often, sometimes with the comma before that is, and sometimes not.
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Snarfsometimes with the comma before that is
Yes, if what follows is a fragment.
What about this?

That is, the day after the night before.

Is that a sentence fragment or a standalone sentence? And this:

That is, they wanted to see how she worked.

Is that independent?
That is is not a subordinator, so just ask yourself this question: Is "They wanted to see how she worked" an independent clause?

And does the day after the night before look like a sentence to you?
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Aspara GusThat is is not a subordinator, so just ask yourself this question: Is "They wanted to see how she worked" an independent clause?
Definitely. So I guess That is there is like using In fact, but with a different meaning. Neither are subordinators.

So the following:

They wanted to see what she was doing, that is, they wanted to see how she worked

is a comma splice, since the part after that is has a predicate, subject and verb.
Yes indeed.