Grammar: Cum + subjunctive
When the Latin word "cum" is used as a conjunction, it usually has some kind of temporal meaning. Sometimes it defines the time ("when"), but it can also be used to describe the time in a variety of ways ("since," "while", etc.). If it is used to define the time at which something happened -- time "when" -- cum is used with an indicative verb.
Temporal cum. When cum is used with the imperfect or pluperfect
subjunctive, it can describe the circumstances that accompanied the main
verb (imperfect subjunctive) or preceded the main verb (pluperfect subjunctive).
» Cum esset iuvenis, Marcus miles erat. When he was a young man, Marcus was a soldier.
» Cum Marciam vidisset, Marcus territus est. When he saw Marcia, Marcus was terrified.
Causal cum. Cum can be used with the subjunctive to explain causal circumstances.
» Cum valde fortis Samson sit, nemo eum superare valeat. Since Samson is very strong, no one can defeat him.
Concessive cum. Cum can be used with the subjunctive to explain contrary circumstances.
» Cum Marcia multam pecuniam habuisset, felix non erat. Although Marcia had a lot of money, she wasn't happy.