The sentences below both sound natural to me, but I can't understand why one usesis and the other was. Can someone please explain this to me
1. I wouldn't say he is clever.
2.I wouldn't say he was clever
"Is" refers to the present, and "was" to the past. If you mean to refer to a lasting characteristic (non-cleverness) of a living person, use "I wouldn't say he is clever." If you mean to refer to a characteristic that was possessed by a person who is no longer living, use "I wouldn't say he was clever."
You can also use "I wouldn't say he was clever" if you are talking about a characteristic that he manifested during a circumscribed period of time in the past, say, while he was at a party where you observed him and were able to form an opinion as to his cleverness or lack thereof.