Here are some expressions you can use to agree and disagree. You will need to use these expressions in a discussion activity.

Agreeing

That’s right!
Absolutely!
Exactly!
Me too!
Yes, I agree!
I totally agree!
I couldn’t agree more!
I see exactly what you mean!
You're right. That's a good point.

Disagreeing

I don’t agree!
I totally disagree!
Absolutely not!
That’s not right!
I’m not sure about that.

Partly agreeing

I agree up to a point, but ...
I see your point, but ...
That’s partly true, but ...
I'm not so sure about that.
 

It is always a good idea to justify your opinions. Don’t just say ‘I agree’, but say ‘I agree because I think that ... (explain your reason).’