When to use 'understand' and when to use'understood'?

[angelatay]

Hello, this is the first time i'm here..so please tell if there's any grammatical error i have made, thanks. there's something that makes me a bit confusing.. Actually which one of the following is true?
1. ...Yes,I understand.
2. ...Yes,I understood.
And if someone asks whether I understand something or not, I should reply 'understood' or 'understand'? because i think if i said 'understood', that means now i don't understand already because it's past tense word.
So please explain this for me..i'm blur now..

 

[Anglika]

Hello, this is the first time I'm here..so please tell me if there's any grammatical error I have made, thanks. There's something that makes me a bit confused. Which one of the following is correct?

1. ...Yes, I understand.
2. ...Yes, I understood.

If someone asks whether I understand something or not, should I reply 'understood' or 'understand'? Because I think if I said 'understood', that means now I don't understand already because it's the past tense.

So please explain this for me..I'm blur now..

Both are correct.

The question itself might be "Did you understand that?" "Yes, I understand it." [present and continual]

OR

"Have you understood what I said?" "Yes, I understood you, but I don't understand why you said it." [past]