| | Re: Answer "I understand" or "I understood"
If you're talking about the car example, the answer would not be *I see it because as you correctly said, the present simple expresses a permanent event and this isn't. It would be I can see it. In English, when talking about sensory perception (with the verbs see, hear, smell, feel, taste) we nearly always use can :
Speak up! I can't hear you. / I can feel something crawling up my leg! /From my house in London I can see the River Thames. / I can smell something burning! / What did you put in this rice? I can taste apples.
On the other hand, when see is used to mean understand, then the present simple is fine because "understanding" is a permanent event :
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. or OK, I see.