1. He can not solve the math problem.

2. He cannot solve the math problem.
3. He can't solve the math problem.
4. He could not solve the math problem.
5. He couldnot solve the math problem.
6. He couldn't solve the math problem.

(Are they all grammatical? What about #5? Is it wrong expression?)

#5 is definetly wrong Also Im not sure about number 2.

All are correct except #5.

Yeah 5 isnt correct but the rest are

I'm pretty sure CANOT has two n's in it-

Cannot?

I don't know, I'm not the best and English-

Have a wonderful day- or night!

All of the given sentences are grammatically correct except for sentence #5: "He couldnot solve the math problem." This sentence is not grammatically correct because there should be a space between "could" and "not," like in sentences #4 and #6.

Here's a breakdown of the corrected sentences:

1. He cannot solve the math problem. (This uses "cannot," which is the proper way to write "can not" as one word.)
2. He cannot solve the math problem. (Same as sentence #1, just different punctuation.)
3. He can't solve the math problem. (This uses the contraction "can't" instead of "cannot.")
4. He could not solve the math problem. (Uses "could not" to indicate past tense inability to solve the problem.)
5. He could not solve the math problem. (The corrected version of sentence #5.)
6. He couldn't solve the math problem. (Uses the contraction "couldn't" instead of "could not" to indicate past tense inability.)

So, to summarize, all the sentences are grammatical except for sentence #5, which should be written as "He could not solve the math problem."