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

ご自宅で快適に語学を学べるチャンスをお見逃しなく。経験豊富な語学講師陣の中からお選びいただき、今すぐ最初のレッスンにお申し込みください！