What are the differences between these sentences?

These are subtle differences but there are certainly differences.

This - usually denotes something close by or really specific. E. g. This is my mother. This is the best ice cream.
That - usually denotes something farther away but not necessarily out of sight. It's still specific. E. g. That is my History teacher over there. That is one big octopus!
It - this is a genderless pronoun, but it has a lot of other uses. In the example you gave, you're using the term as a (I believe) part of the predicate. Distance doesn't matter. E. g. I like it. He ate all of it.

Sorry English is so confusing! We have an awfully odd language; a lot of that is because of people bringing it from place to place and then mixing with local cultures and languages. You're doing great!

Thanks to Jes above

Sure, Loner, English must be tough for the esl. Having spent my lifetime on and off in Journ, still I can't always answer (or even approach) such q's

But the good thing is, you don't need a pronoun at all! You can just say
"The meeting commenced at 7, but he didn't know."

The emphasis is less strong, making it sound like just an observation rather than stressing something negative had happened.