The River Nile _ (flow) very fast today - much faster than usual.
We use the present continuous (is flowing) here, because it's something that's happening now and isn't repeated or continuous. However, suppose that for some strange reason the Nile flowed very fast every Wednesday or every tenth Friday. Then we could say "The River Nile flows very fast today".
We usually grow vegetables in our garden, but this year we _ (not/grow) any.
We use the present continuous (aren't growing) because it's something we're doing (or not doing) during a particular period of time. However, suppose that for some strange reason we follow a schedule whereby we grow vegetables in our garden, except that every tenth year, we don't grow any. Then we can say "This year we don't grow any".
The car has broken down again. B: The car is useless. It_.
The present continuous (it's always breaking down) means that we feel as though it's always breaking down, but we don't really mean that it breaks down every time. The simple present (it always breaks down) - although sometimes also an exaggeration - means that it breaks down every time, and is more likely to be used with reference to a series of repeated events ("When we go on holiday, it always breaks down"; "It always breaks down on Thursdays").
Oh I forgot my glasses again. B: That's typical! _.
The present continuous (You're always forgetting your glasses) is a colloquial exaggeration. The simple present (You always forget your glasses) means that, within a specified or implicit series of events, you forget them every time ("When you go to night-school, you always forget your glasses"; "When you're driving, you always forget your glasses").
From Thomson & Martinet, A Practical English Grammar (Oxford University Press, 1986):
He is always losing his keys. This form is used, chiefly in the affirmative:
(1) For a frequently repeated action, usually when the frequency annoys the speaker or seems unreasonable to him. Tom is always going away for weekends (present continuous) would imply that he goes away very often, probably too often in the speaker's opinion. But it does not necessarily mean that he goes away every weekend. It is not a literal statement. Compare with always + simple present: Tom always goes away at weekends = Tom goes away every weekend (a literal statement).
I/we always + continuous is also possible here. The repeated action is then often accidental: I'm always making that mistake.
(2) For an action which appears to be continuous. He's always working = He works the whole time. This sort of action often annoys the speaker but doesn't necessarily do so. He's aways reading coud imply that he spends too much time reading, but could also be said in a tone of approval.
Excuse me. (anybody/sit/there?) _
The present continuous (Is anybody sitting there?) is asking whether the seat is in use now (where "now" implicitly includes instances where the person who was sitting there has just left for a few minutes or will otherwise be using the seat shortly).
The simple present (Does anybody sit there?) could be a way of asking either whether anyone habitually sits there ("Yeah, that's Bob's seat - he sits there whenever he comes in!") or whether anyone ever sits there (imagine the staff planning to rearrange the furniture: "Does anybody sit there?" - "No, hardly ever - we may as well remove that seat so people have more room to walk about").