A. use There ~ when we talk about something for the first time. (e.g. I’m sorry I’m late. There was a lot of traffic)
A1. There also means ‘to/at/in that place. (e.g. The new restaurant is very good. I went there (= to the restaurant) last night.
A2. can say there will be/ there must be/ there might be/ there used to be/ there must have been/ there should have been etc. (e.g. Will there be many people at the party?)
A3. also say there is sure/ certain/ likely/ bound to be ~ (e.g. There is bound (=sure) to be a flight to London this evening.

B. It = a specific thing, place, fact, situation, etc. (e.g. I wasn’t expecting them to come. It was a complete surprise. (It = that they come)
B1. use it to talk about distance, time and weather.

C. I don’t like this town. There’s nothing to do here. It’s a boring place.