“We can talk about plans using the present progressive…
… We use going to rather than will for plans, decisions and firm intentions.”
(Market Leader Business Grammar and Usage, two lines apart from each other on page 28)
I don’t know why this ties people up in knots so much when the basic explanation is quite simple:
Present continuous for arrangements, meaning something that has already involved someone else e.g. because I have fixed a time with someone, booked something or bought a ticket
Going to for plans, meaning something that I have decided
Will for predictions, meaning my imagination about something I can’t affect, e.g. fate, destiny and the stockmarket
A nice reinforcement of that is saying arrangements are in my diary, plans are in my head and predictions are my imagination about things that are out there in the world.
It obviously gets a bit more complicated if they bring up going for predictions with present evidence and will for spontaneous intentions, but those can easily be added onto this general explanation (as they neither fit into it nor spoil it). Or at least so I have found numerous times over the last 8 years or so since I first typed up a worksheet on the topic. More recently I’ve been trying to work on something using the fact that “I’ve arranged to…” means the same as “I’m…ing” etc to bring those explanation words into the lesson more naturally, but the big hole at the moment is that “I intend to/ plan to…” seems to be a lot less certain than “I’m going to + inf”. Will let you know how it goes after I try it in class for the first time tonight.
A more detailed examination of this here:
General explanations of English future forms – NEW LINK
Games ideas for a mix of tenses here:
Activities for mixed future tenses – NEW LINK
Some worksheets contrasting them and using them together are included on this page:
TEFLtastic 