Which of the following is ungrammatical:

The problem is no one knows the truth.


The problem is , no one knows the truth.

? And if both are grammatical, which one would be more formal?

  • 2
    Just don't say "The problem is, is that no one knows the truth." Very common mistake that native speakers make. Only one "is" is needed. – Mixolydian May 30 '19 at 17:33

The full version of that statement would be

The problem is that no one knows the truth.

In your sentences, the word "that" has been dropped. It is quite acceptable to drop the "that" in cases like this, and it doesn't make the sentence ungrammatical. I guess you could regard both of your two sentences as slightly less "formal" than the version with "that", but there isn't any difference in formality between your two examples.

As far as punctuation is concerned, "The problem is that no one knows the truth." does not require a comma, so none should be needed when you drop the "that". However, rightly or wrongly, I would include a comma anyway, as in your second choice,

"The problem is, no one knows the truth."

just to make it clearer to parse while reading. The Longman dictionary online seems to agree, since they give as an example the following:

The problem is, there isn’t enough time.

Short answer: they are both grammatical and equally formal/informal.

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