Agreeing and disagreeing

1. Agreement

1.1. Agreement with positive

A. Yes + subject + auxiliary/BE. For example: Yes, I do | Yes, I am.
B. Yes, (definitely).
C. I quite agree.

Example:

a. Sam sings very well, don't you agree?
b. I'm sure you're right.
c. Yes, he does.
d. Yes, absolutely!

When we want people to agree with us, we often use Tag Questions like isn't he? and wasn't it? etc. For example:
a. It's very cold today, isn't it?
b. Yes, it's awful!

Note: When asking for agreement, we use a falling tone, not a rising tone. [See Tag Question for more details.]

1.2. Agreement with negative

A. No + subject + auxiliary/BE + n't. For example: No, I don't | No, I'm not.
B. No, (definitely not).
C. I agree (that ... not ...).

Example:

a. Programming languages aren't very hard to learn, are they?
b. Definitely not.
c. They certainly aren't.
d. I agree (that they aren't).
e. No, I'm afraid that they aren't.

2. Disagreement

It's more polite to disagree partly than wholly.

A. Yes, but (on the other hand) ...
B. True, but (then) ...
C. I'm afraid I disagree (with you).

Example:

a. The government tax law is unfair, don't you think?
b. Well, it's unfair to some people perhaps.
c. True, but then no tax is perfectly fair.
d. Yes, but (on the other hand) it hits the rich rather than the poor.
e. I'm afraid I disagree with you.