Student: Why are you on cloud nine? Youdon't laugh often.

Correct: Whydon't you laugh often when you are on cloud nine?

Explanation: Cloudnine means 'a state of perfect happiness', so your sentences arecontradicting.

Student: That's why you are so happy?

Correct:Is that the reason why you are so happy?

Explanation: 'That'swhy you are so happy' is a statement and placing a question mark atthe end doesn't justify that it's grammatically correct as aquestion.

Student: It isn't showing off but'

Correct: Iam not showing off but...

Explanation: Thecat itself is not showing off that she's expensive, but I think theperson who's talking is referring to himself.

Student: I'm envy of you!

Correct:I envy you! / I am envious of you!

Explanation:'Am' is a linking verb and it should be followed by anoun/adjective; in the correct sentence, 'envious' is the adjectivedescribing himself. You can also say 'I envy you'; with 'envy' as averb which is something that he feels or do.

Student: She is really nice cat.

Correct: Sheis a really nice cat.

Explanation: Use'a' as the determiner for the noun 'cat'.