Present and Past Mixture?

[Leandro-Z]

Hello... I would appreciate it if you could explain me the following:

Which of the sentences below is correct? Which one should I choose? Why?

I misunderstand the instructions you gave us.
I misunderstood the instructions you gave us.

I know they have different meanings, I must have made a different question: Is it okay to say "I misunderstand the instructions you gave us? Why?

I ask that because my teacher told me that you can`t mix a past tense with a present tense. Please, I am looking forward to receiving coherent answers. Thank you in advance for your help and effort!

 

[Raymott]

Hello... I would appreciate it if you could explain me the following:

Which of the sentences below is correct? Which one should I choose? Why?

I misunderstand the instructions you gave us.
I misunderstood the instructions you gave us.

I know they have different meanings, I must have made a different question: Is it okay to say "I misunderstand the instructions you gave us? Why?

I ask that because my teacher told me that you can`t mix a past tense with a present tense. Please, I am looking forward to receiving coherent answers. Thank you in advance for your help and effort!

No, that sentence is strange. If you misunderstand something, you are not likely to know about it. If you do find out that you've misunderstood it, you no longer misunderstand it. However, there's nothing wrong with, "I don't understand the instructions you gave us," which is probably what you mean.

You certainly can "mix a past tense with a present tense". For each clause in a sentence, you need to choose the appropriate tense.
In your correct sentence in blue above, you have gone from present (I ask) to past (my teacher told me) to present again (you can't).