Understood what you said/ have understood what you have said

Question

If a teacher is talking to me and what should I say between:

• I understood what you said
• I have understood what you have said

Answer 1

I understood what you said to me yesterday/last week.

I have understood what you have (just) said to me.

Couldn't be easier.

Answer 2

I wouldn't say either of those things; to me the idiomatic choice would be "I understand what you said".

I would say this because

• the understanding is still present
• the speaking is in the past