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?

What makes you think it should be "it"? :)