Cum meaning 'when'
cum + an indicative verb is used to express a temporal clause:
cum vinum bibit, laetus est.
When he drinks wine, he is happy.
It can also be used to express the circumstances under which an event took place, as well as the time. For the present and future, this is formed with cum + indicative:
cum domum redit, cenit.
When he returns home, he dines.
There is no clear difference between this and the temporal use.
The difference is more obvious in the past use, when cum is used with the subjunctive:
cum vinum biberet, laetus erat
When he drank wine, he was happy (but with the force 'under the circumstances in which he could drink wine').
The indicative, however, will still be used if the cum clause contains the main idea of the sentence.
