Can someone explain how can we distinguish between a conditional and a subjunctive mood?

We would start the meeting if Sam were here.
vs
We would start the meeting if Sam was here.

Also, what are the rules that determine the usage of IF/WILL/WOULD/WERE/COULD/IS/WAS etc.?

Below is one such question?

Prior to the battle of Actium, Anthony's generals argued that if Cleopatra goes home many of the Roman senate, Roman people and Roman army would quit their support of Octavianus.

A) goes home many of the Roman senate, Roman people and Roman army would quit their
B) would go home many of the Roman senate, the Roman people and the Roman army would quit their
C) would go home many of the Roman senate, the Roman people and the Roman army will quit their
D) went home, then many of the Roman senate, the Roman people and the Roman army will quit their
E) were to go home many of the Roman senate, the Roman people and the Roman army would quit their

OA E