Here is a list of useful phrases on agreeing and disagreeing from the simplest to the most complex ones:
• Yes.
• True.
• That’s right.
• I don’t think so.
• I can’t agree.
• Oh, surely not.
• True enough.
• I absolutely agree.
• I couldn’t agree more.
• I’m not at all sure, actually.
• I disagree, I’m afraid.
• That’s not the way I see it.
• There’s a lot in what you say, but …
• I am of exactly the same opinion