Es imprescindible que hayamos entendido esta lección.
It is essential that we have understood this lesson.
I don't know if the English translation above is grammatically incorrect, but it doesn't sound like something a native English speaker would say. A better translation might be:
'It is essential that we understand this lesson.'
It's not a literal translation of the Spanish, but translations between languages often can't be if you want to convey the accurate meaning.
If the emphasis is on the completion of the understanding, perhaps the translation could be:
'It was essential that we understood this/that lesson.'
But here we're using the past tense instead of the present tense in the Spanish sentence.
