1) ashita wa donna tenki desu ka?
1) ashita wa kumori aru to omoimasu
I'm not sure about "kumori aru" it sounds strange in my ears and might as well sound strange for a native speaker, gotta wait for Toritori to make sure.
You can say,:
ashita wa kumori dato omoimasu.
I guess its more neutral.
3) Anata no nihongo no sensei wa, konshuu no shuumatsu, nani wo shimasu ka?
3) eigo wo mite to omoimasu
use the jisho-kei in this situation:
"eigo wo miru to omoimasu"