omission of that

JungKim(1) Is this sentence grammatical?

No. One too many thats.

JungKim(2) What if I take out the first that and keep the second one?

Not very nice. I don't recommend it.

JungKimAnd vice versa?

Very nice.

JungKim(3) Can I take out both of them?

To find out why it is ungrammatical, just discard the if-clause:

"I thought that that things would go back to the way they were."

As you can see, the clause that complements "thought" now has two subordinators introducing it and is thus ungrammatical.

BillJ

"I thought that, if I got them under the same roof, that things would go back to the way they were."
(1) Is this sentence grammatical?
(2) What if I take out the first that and keep the second one? And vice versa?
(3) Can I take out both of them?

CalifJimJungKim(1) Is this sentence grammatical?No. One too many thats.JungKim(2)

JungKimI posted this question because I remember coming across some native speakers putting the second that quite consistently.

Very observant. That does happen in speech, but it shouldn't happen in writing if people take the time to proofread what they write.

The second 'that' is actually just a repetition of the first 'that' in these cases, and not

CalifJimThe second 'that' is actually just a repetition of the first 'that' in these cases, and not a new 'that', if you see what I mean. It's as if the speaker has said something parenthetical and is picking up where he left off before the parenthetical material. He signals the fact that he's doing this by the repeating a word or two from the end of the non-parenthetical