In some respect, "cum" is similar to the English conjunction "as", which has quite a range of meanings, and at times seems to be using many of them all at once.

There are two categories of meanings for "cum":

i) Strictly temporal (when)

When "cum" is temporal, the mood of the verb in its clause is often indicative.

It is almost always indicative when the tense of the verb in the main clause is present or future tense. When the tense of the main verb is one of the past tenses, then the mood of the "cum" clause is most often subjunctive.

Cum responderit [future perfect], omnia intellegetis.
(When he answers (will have answered), you will understand everything.)

ii) Circumstantial (as, whereas, since, because, although)

It sets a backdrop for the action in the main clause without spelling out what the relationship is between them. A circumstantial "cum" clause can be translated as "since", "because", and "although" (causal and concessive).

When the "cum" clause is circumstantial, the mood of its verb is always subjunctive and observes the sequence of tenses.
　

Contemporaneous time - Subjunctive Imperfect

Cum responderet, non aderatis.
(When he was answering, you were not present.)

Time prior - Subjunctive Perfect

Cum responderit, omnia iam intellegitis.
(Because he answered, you now understand everything.)

Time prior - Subjunctive Pluperfect

Cum respondisset, omnia intellexistis.
(Because he had answered, you understood everything, or "When he had answered..". or "Since he had answered”..)

Concessive - Subjunctive Pluperfect

Cum respondisset, non tamen intellexistis.
(Although he answered, you nevertheless did not understand.)