If I talk to you about something.
Heh, remember the going to the concert, last week.
We can then refer to it as "that". A previously mentioned object.
Oh, yeah, I remember that.
The question is interesting. I can't immediately see a rule for when you can use "that" and when "it" is better.
Clearly, when "it" is not simple, "that" will work.
Remember all those hours of study you had to do to get fluent.
"it" does not seem correct for a long process. Though you could stil say "I rememeber it".
"I remember that." somehow seems more appropriate.
Any more clear about that?