It or that?

[Mr Francis]

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"?
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You can get it if you really want.

[Alan]

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
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English as a Second Language
You can read my ESL story Present Simple