agree vs. agreed

English as a Second Language English Vocabulary, Grammar and Idioms
1

Hi.

A: I think we should go home now.
B:Agreed.

Q1:Why is it “agreed” but not “agree”?
Q2:But, if B says “I agree” instead of simply just “agreed”, what’s the difference? Or “I agreed” is correct expression in this context?

Thanks.

2

Q1-- The response is idiomatic. It was shortened long ago, I suppose, from ‘It is agreed’.
Q2-- ‘I agree’ = ‘Agreed’. ‘I agreed’ is wrong.