第一句: I don't agree.
A: We should move to a larger house.
B: I don't agree. I think our present house is quite comfortable.
第二句: I don't think so.
A: Do you think it will be a fine day tomorrow?
B: I don't think so.
I'm against it.
I'm strongly opposed to your idea.
I don't see it that way.
I have a different opinion.