"There were two thousand deaths due to the war."
Translation:Οι θάνατοι εξαιτίας του πολέμου ήταν δύο χιλιάδες.
Υπήρχαν is imperfect, υπήρξαν is aorist. There is a lexeme for that (in the imperfect tense) in the new tree. One can't really tell the difference in English, as both translate to "There were...", and they can often be used interchangeably in Greek (depends on the sense of duration one wants to convey, so usually both are correct), so "υπήρχαν" does it for both.
Thanks D.. Obviously I wasn’t very clear, I was asking why it had to be οι θάνατοι εξαιτίας του πολέμου ήταν δύο χιλιάδες and that οι θάνατοι ήταν δύο χιλιάδες εξαιτίας του πολέμου was incorrect. I was wondering was there a rule that governed the construction of the sentence.