It or that?

Hello to everyone,

I’ve got a problem. I haven’t still understood when using “it” and when using “that”. I think there are cases where is possible to use “it” or “that” indifferently. Am I wrong?

Many thanks in advance,
Francesco

PS: Please tell me if I did any mistakes in my email
PPS: Could I also say “Tell me if I’VE DONE any mistakes”?

Hi Francesco,

In this sentence: I didn’t understand it - this refers to something in general

In this sentence: I didn’t understand that - this refers to something particular

We make mistakes.

Alan