That’s OK. Forget it! / Forget that!

Question

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?

Answer 1

"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...

Answer 2

"Forget it" is an idiom. It means "let's stop talking about this," usually in an aggressive or passive-aggressive way. Seeing it used in a neutral way like this is unusual, and would require the speaker to make it very clear in their tone if they're not angry, but it's not completely unheard of. (On the other hand, maybe they are still mad. Hard to tell.)

"Forget that" is less idiomatic, but related. Rather than a general expression of frustration, I read it as a subtle scorn on the idea that an apology is needed—a friend's firm assurance that everything is forgiven, probably followed by an abrupt transition to a more lighthearted subject: "Forget that! Let's go get ice cream." Mind that it is still mildly aggressive, so you would not want to say this to someone who wasn't a close friend who would definitely understand the intended meaning.