Actually these are two questions in one. I want to express that once something was done, then something else became possible. The first question is, how to say that if that actually happened, and the second when it never happened but could have happened.
Is this correct?
Once the Germans crossed the river Dnepr, they were able to encircle the Soviet forces.
What about this?
Once the Germans would cross the river, they would have been able to encircle the Soviet forces.
Based on the information you have given, I believe you mean to express that
the Germans could not encircle the Soviet forces until they finished crossing the river.
Your first example communicates this. Your second example, however, seems to be aimed at a subjunctive conditional set in the past:
Should the Germans have crossed the river, they would have be able to encircle the Soviet forces.
If you wanted to say "they really did it and the consequences really happened"(like a piece of history), it would be the first sentence with a fix:
Once the Germans had crossed the river Dnepr, they were able to encircle the Soviet forces.
The first part is Past Perfect because it is the action before the second part (Sequence of tenses). Now it is grammatically correct.
A sentence with meaning "if-then" should be created with Third Conditional (if something unreal in the past -> something unreal in the past) (bold text shows the difference compared to your second sentence):
If the Germans had crossed the river, they would have been able to encircle the Soviet forces.
The formula is: If + Past Perfect (in some cases Past Perf Cont, definitely not this one) + would/should/could/might + Verb 3 form (be-was-been).
Once the Germans crossed the river Dnepr, they were able to encircle the Soviet forces.
is OK. "once" introduces a completed action after which another one occurs.
Once the Germans would cross the river...
is not grammatical. There is no completed action after once, only a possibility. To say "when it could have happened", then if/then is better:
If the Germans crossed the river Dnepr, then they would have be able to encircle the Soviet forces.
If it never happened:
The Germans never crossed the river Dnepr, so they weren't able to encircle the Soviet forces.
To convey the action never happened, but this is how it could have happened:
Had the Germans crossed the river Dnepr, they could have encircled the Soviet forces.
First part of the sentence use Past Simple. Past Simple tense need a time expression that fixes the action at a ceartian time in the past.
For e.g.
One the Germans crossed the river Dnerp in 1942, they were able to encircle the Soviet forces
But as @user3169 already said, you are describing two events that happend in the past the past where one event certainly happened before the second one. In this case we use Past Perfect - for the event which occurred first and Simple Past for the second one.
