[susan53]

1. Generally, the present - because it's true that you understand at the time of speaking:
A : I'm sorry, but I won't be able to help because I have to visit my grandfather in hospital.
B : OK, I understand.

Using the past tense here would refer to previous understanding, and might sound irritated : Yes, I understood fifteen minutes ago, the first time you told me. Don't go on about it

2. Either. You might shout more or less simultaneously with the action - Oi! That's my foot! or say it afterwards, seeing it as a past action - That was my foot which you just kicked!

3. I'd probably say Can you see ... rather than Do you see... but it's still the present. In any case, if the car is still there, the past is logically impossible. Even if you were standing in front of it, it would still be visible With a completely obscured object you would use the past though - eg in a memory test : I lay out five playing cards on the table and then turn them face down and ask Did you see the three of hearts? Here the action of seeing the card is past, because I've obscured them. So the past verb is logical.