Although I know the general rules when to apply which tense, I'm often confused and do not really know which one to use. I can find pros and cons for each tense.
Two examples: In terms of spending vacation/holidays and having a doctor appointment, which tense is most natural? When is it OK to go with another tense?
I am especially unsure about the will-future. I've seen/heard something like "I will see the doctor at 4PM". This sounds like a fixed arrangement / schedule, but arguments for will-future are "spontaneous decision", "promise", or "opinion/hope". Is this incorrect use of non-natives? If not, what is the reasoning for saying it like that?
Additionally to this question, is my general understanding (see below) correct? Did I miss any arguments pro/con a particular tense and which one?
So, here's how I understand it:
If I've already booked a flight or have made an appointment, respectively, it's a (personal) arrangement, thus present continuous:
Next month, we're spending our vacation/holidays in Canada.
I'm seeing the doctor tonight.
Technically, vacation/holidays and doctor appointments are scheduled, thus present tense. On the other hand, I think I haven't seen any uses of present tense in that case. Is the key difference between an arrangement that I made myself and a schedule made by someone else? That is, I chose myself to do my vacation at this time as opposed to someone else scheduled my vacation.
I would use present tense, though, when I give the exact time as I give the exact schedule. Or is present tense wrong altogether1?:
We spend our vacation/holidays in Canada from September, 1st to the 12th.
I see the doctor at 6 o'clock.
If I haven't taken any actions but it's decided, it's my/our intention, thus going-to-future:
Next year, we're going to spend our vacation/holidays in Canada.
I'm going to see the doctor soon.
If I/we haven't made any decision, let alone take any actions, it's a prediction/opinion/hope, thus will-future:
(I think,) We will spend our next vacation/holidays in Canada.
(I think,) I will go to the doctor.
Another reason for will-future is when I make a promise, or a spontaneous decision:
Yes, we will spend our next vacation/holidays in Canada.
Yes, I will go to the doctor.
1In respect to arranged future events. I know that, for instance, the sentence "I will do that when I see the doctor" is correct. But that's another story.