Sentence structure

No.

I don't know when I'll be in New York next.

PreciousJonesDoes this sentence make sense?

Yes, but it’s ungrammatical. I’d say I don’t know when I’ll be in New York again or maybe I don’t know when my next visit to New York will be.

Rover_KENo.I don't know when I'll be in New York next.

I don't understand why I can't use the sentence I provided?

Can you say:

When is the next time you'll be in town?

Aspara GusYes, but it’s ungrammatical. I’d say I don’t know when I’ll be in New York again.

How is it ungrammatical?

PreciousJonesI don't understand why I can't use the sentence I provided.

For the same reason you can’t say *I don’t understand why can’t I use the sentence I provided.

It's a very awkward sentence. Here are some alternatives:
I do not know when I will next be in New York.
I do not know when I will be in New York again.
I do not know when my next trip (or visit) to New York will be.

Aspara GusQuote

Frankly, to my ear - " I don't understand why can't I use the sentence I provided." sounds fine...

Like if someone said, "I don't understand... why can't I use the sentence I've provided." I wouldn't think it was wrong or ungrammatical.

Correct me if I'm wrong.

Thank you!

I don't understand. Why can't I use the sentence provided?
I don't understand why I can't use the sentence provided.
Do you see the difference?

PreciousJones"I don't understand... why can't I use the sentence I've provided."

This is quite different. Here, you have two sentences: I don’t understand. Why can’t I use the sentence I’ve provided? The latter sentence is a direct question, so the word order is correct. In the original, I don’t understand why I can’t use