They were fighting Nazis for three days.
They fought Nazis for three days.
The first sentence is said to be the correct usage of past progressive tense. But why can't we use it in the same manner in present progressive tense?
It would be wrong to say:
We are fighting Nazis for three days?
The correct form is:
We have been fighting Nazis for three days.
Why do they differ in past progressive and present progressive?