As I understand it, the first "that" is a response on the first sentence, that's why there is "that's OK", and not "it's OK". But why after that is used "it" – "Forget it!"? Can I answer on the first phrase as it was answered in the second and in the third answers.
‘I’m sorry I was angry yesterday.’ ‘That’s OK. Forget it!’
‘I’m sorry I was angry yesterday.’ ‘That’s OK. Forget that!’
‘I’m sorry I was angry yesterday.’ ‘Forget that!’
P.S.
From Longman dictionary:
"That" – Used to refer to a person, thing, idea etc that has already been mentioned or is already known about.
"It" – Used to refer to a thing, animal, idea etc that has already been mentioned or is already known about.
How is it possible to choose "that" or "it" in this case?
Best Answer
"Forget it" is an idiom. You are not actually telling the person to lose their memory.
"Forget that" isn't the idiom, so it sounds like a weirdly aggressive command.
In these idiomatic uses of pronouns, I'm not even sure what "That" or "it" actually refer to...