Posted by rfvv on .
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?)

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

English 
Writeacher,
All are correct except #5.