"우리는 이제 이해했습니다."
Translation:We now understand.
This may be a bit difficult to explain (although I have done it for other verbs that are used similarly)…
It is in the past tense because the fact that you are understanding it now is due to you having a moment of insight at some point in the past. Think of the past tense as referring to having experienced that moment of insight. By having that moment, you are now continuously understanding in your present state. The Koreans to not refer to this state, but to that moment in the past.
(1) Get it (Understand) vs Have got it (Understood)
우리는 이제 이해합니다 = We get it now
우리는 이제 이해했습니다 = We've got it now
In English, "Have got it" & "Get it" can be used interchangeably and amount to the same thing.
In Korean as well,
이해하다 ~ 이해했다 = get it / have got it = understand.
(2) be understood (passive) = 이해되다