How do you distinguish between “I understand” and “I understood”? What’s the difference?

Question

How do you distinguish between “I understand” and “I understood”?  What’s the difference?

Answer

If you wish to state that at the present moment you understand, then you have two choices:
1) I understand
2) I have understood  (the present perfect describes the present, not the past) 
If you wish to state that you understood at a past moment, then say
3) I understood.

Answer

“I understand” is present tense, and “I understood” is past tense, used if you are recounting [describing] an event that has now finished.
'I understand what you mean'  [present]
'I understood what he meant'   [past, now finished]

Answer

I understand is present tense 
I understood Is past tense