2. How Best to Agree and Disagree
Agree or Disagree with the Idea Not the Person

A. How to Agree Strongly with an Opinion.

1.   I couldn't agree more!
2.   That's absolutely true!
3.   Absolutely!
4.   I agree with your point.
5.   I'd go along with your point.
6.   I'm with you on that point.
7.   That's just what I was thinking.
8.   That's exactly what I think.
9.   That's a good point.
10. That's just how I see it.
11. That's exactly my opinion.
B. How to Half Agree with an Opinion.

1.   Yes, perhaps, however ...
2.   Well, yes, but ...
3.   Yes, in a way, however ...
4.   Hmm, possibly, but ...
5.   Yes, I agree up to a point, however ...
6.   Well, you have a point there, but ...
7.   There's something there, I suppose, however ...
8.   I guess your idea could be right, but ...
9.   Yes, I suppose so, however ...
10. That's worth thinking about, but ...
C. How to Disagree Politely with an Opinion.

1.  I am not so sure.
2.  Do you think so?
3.  Well, it depends.
4.  I'm not so certain.
5.  Well, I don't know.
6.  Well, I'm not so sure about that.
7.  Hmm, I'm not sure your idea is right.
8.  I'm inclined to disagree with that idea.
9.  No, I don't think so.
D. How to Disagree Strongly with an Opinion.

1.   I disagree.
2.   I disagree with your idea.
3.   I'm afraid I don't agree.
4.   I'm afraid your idea is wrong.
5.   I can't agree with your idea.
6.   I couldn't accept that idea for a minute.
7.   You can't actually mean that.
8.   I wouldn't go along with your idea there.
9.   You can't be serious about that point.
10. You must be joking.
11. It's possible you are mistaken about that.

You need to be very polite when disagreeing with someone in English, even someone you know quite well.
With someone you know very well, you can disagree more directly.