1). Whereas the present continuous in the future tends to talk about definite arrangements that have been made (or to ask about arrangements), the present simple describes events that are timetabled.
Compare the differences below:
As you can see, the present continuous is only speaking of definite events (we’re having dinner together etc.), and asking about definite future events, whereas the present simple talks about events that are scheduled for a particular time (the table is at 7 etc.), or asks about them.
2). When a time is put into the sentence, the present progressive can be used to express arrangements and plans. ‘Will’ cannot be used for plans and arrangements, but ‘going to do something’ can:
We’re having dinner together on Thursday; I’m travelling to Vietnam next Saturday; what are we going to do tomorrow?
3). The present progressive can also be used to insist that something is or isn’t done or to refuse something such as:
You’re having a drink whether you like it or not!; I’m not buying that!; there’s no way we’re being talked to like that
Unlike the present simple, which when used with ‘why don’t you…?’ is used to suggest something:
Why don’t you go home?; why don’t you eat more food?
4). The present progressive can also be used to say something you are just about to do or just before you do it:
I’m going to bed now; I’m coming now, just a second
Which, again, is unlike the present simple.
The present simple instead tends asks for or gives instructions:
Where do I buy apples tomorrow?; tomorrow, please walk to the shop and buy some milk