1) CUM meaning "WHEN" (or "while") - CUM "TEMPORAL".
A 'Temporal' clause is one to do with 'time'. These meanings of 'When' or 'While' are the only ones that aren't necessarily always followed by the subjunctive. This will depend on the time referred to in the sentence.
A) Present or Future Time
CUM is followed by a normal INDICATIVE verb.
e.g. CUM IN URBE AMBULAS, SCELESTOS CAVE!
When you are walking in the city, watch out for crooks!
CUM EIUS VOCEM AUDIO, DELECTATUS SUM.
When I hear her voice, I am delighted.
CUM ROMAE ESTIS, AGITE EODEM MODO SICUT ROMANI
When in Rome, do as the Romans do
Don't be caught out by the use of the 'LOGICAL FUTURE' however:
e.g. CUM ILLUM SERVUM CAPIAM, EUM VERBERABO
When I catch that slave, I shall beat him
The Romans (as in most other languages apart from English!) were much more precise in their use of tenses than we are: I haven't found that slave yet - only in the future when I do (or will do!) will I be able to beat him…!
B) Past Time
In Past time however, CUM is followed by the IMPERFECT or PLUPERFECT SUBJUNCTIVE
e.g. PUER, CUM PROPE FLUMEN AMBULARET, IN AQUAM CECIDIT.
When/While the boy was walking near the river, he fell into the water.
HANNIBAL, CUM AD MONTES ADVENISSET, COPIAS ALLOCUTUS EST.
When Hannibal arrived at the mountains, he addressed his troops.
Notice that although the verb in Latin is in the Subjunctive, it is translated by the equivalent English INDICATIVE; the Pluperfect Subjunctive can also sometimes be better translated by an English AORIST tense, as here: 'When Hannibal ARRIVED…', not 'When Hannibal HAD arrived…".
Another useful thing to remember is that if you see CUM with any other tense of Subjunctive than Imperfect or Pluperfect, it cannot mean "When".
2) CUM meaning "SINCE" - CUM "CAUSAL"
With this meaning ('since' in the sense of 'because' - hence the technical name!),CUM is always followed by the Subjunctive, in any tense. The tense used will correspond to the same tense of the INDICATIVE that you would naturally use to translate it in English.
e.g. CUM AB URBE IAM DISCESSERIS, HANC EPISTULAM MISI.
Since you have already left the city, I have sent this letter.
(Perfect Subjunctive translated by a normal English Perfect tense)
CUM AD THEATRUM MECUM VENIRE NOLIS, SOLUS IBO.
Since you don't want to come to the theatre with me, I'll go on my own.
(Present subjunctive translated by normal English Present.)
A "traditional" example, often seen in Latin literature, is:
QUAE CUM ITA SINT/ESSENT, ………..
Since this is/was the situation……
(literally: 'Which things, since they are/were so, ……)
It is often difficult to decide between whether to translate CUM as 'When' or 'Since': both can sometimes seem to make equal sense:
e.g. CUM HOSTES URBEM OPPUGNARENT, CIVES TIMEBANT.
When the enemy were attacking the town, the citizens were afraid
or: Since the enemy were attacking the town, ……..
In such situations you have a "free choice" as to which to use! This of course explains why the Romans used this same word 'CUM' having the sense of both our English words.
Remember, though, that CUM followed by the Present or Perfect subjunctive does not mean 'When'. This is at least one way that can help decide between the possible meanings.
3) CUM meaning "ALTHOUGH" - CUM "CONCESSIVE"
Yet another meaning of CUM is 'although'. This is much less common than the others. Once again, with this meaning it is always followed by the Subjunctive.
Because it has such a distinct sense, you will tend to 'know it when you've got it': it will generally be clear from the context of the sentence that neither "when" nor "since" conveys the correct meaning; basically, if the other two meanings don't make sense, try "although"!
For example, supposing you see:
CUM AMICUS SIS, TE IUVABO.
This is CUM + Present subjunctive, so it can't mean 'When'.
Try 'SINCE': Since you are my friend, I will help you.
This works perfectly well, and is obviously the correct choice.
CUM AMICUS SIS, TE NECABO.
'Since you are my friend, I shall kill you….' - I don't think so!
Try 'ALTHOUGH': 'Although you are my friend, I shall kill you.'
OK, still a bit weird, but at least it does make sense of a sort….!
CUM + INDICATIVE: MUST mean WHEN or WHILE.
CUM + Present or Perfect SUBJUNCTIVE: SINCE or ALTHOUGH.
CUM + Imperfect or Pluperfect SUBJUNCTIVE: WHEN
Try the meanings in that order and decide which makes the best sense.
Try to decide whether you have CUM used in a Temporal, Causal or Concessive Clause, and then translate it appropriately. Use your cursor first to highlight the correct type of clause (there may be more than one answer),and then to see whether you have translated it correctly.
1. CUM MILITES CASTRA POSUISSENT, MURUM AEDIFICAVERUNT.
Answer: When the soldiers had pitched camp, they built a wall.
2. CUM IN URBE AMBULAREMUS, AMICOS VIDIMUS
Answer: When we were walking in the city, we saw our friends.
3. CUM FILIUS MEUS PROCEDERE NON POSSIT, EUM PORTABO.
Answer: Since my son can go no further, I shall carry him.
4. SENEX, CUM PUEROS IN HORTO CONSPEXISSET, CANES LIBERAVIT.
Clause: Could be Temporal or Causal (but I'd say Temporal!)
Answer: When the old man spotted the boys in his garden, he released the hounds (!)
5. CUM HOSTES OPPIDUM OPPUGNARENT, CIVES IN VILLIS MANEBANT.
Clause: Concessive (could be Temporal)
Answer: Although the enemy were attacking their town, the citizens stayed in their houses.
6. CUM REX NECATUS SIT, QUIS NUNC REGET?
Answer: Since the king has been killed, who will rule now?
7. DUX, CUM CONSILIA HOSTIUM COGNOVISSET, LEGIONES PARAVIT.
Clause: Temporal (could be Causal)
Answer: When the general got to hear of the enemy's plans, he prepared the legions.
8. CUM PECUNIAM INVENIEMUS, DIVITES ERIMUS.
Answer: When we find the money, we'll be rich.
9. CUM PUELLAM MAXIME AMEM, EAM IN MATRIMONIUM NON DUCAM.
Answer: Although I love the girl very much, I'm not going to marry her.
10. CUM GRAVISSIME HASTA VULNERATUS ESSET, AD CASTRA REDIRE NON POTERAT.
Answer: Since he'd been terribly wounded by a spear, he couldn't return to camp.
11. GRAECI, CUM TROIAM CEPISSENT, DOMUM NAVIGAVERUNT.
Answer: When the Greeks had captured Troy, they sailed home.
12. EPISTULAM TUAM CUM ACCEPISSEM, AD FORUM STATIM FESTINAVI.
Clause: Temporal or Causal
Answer: When I received/Since I had received your letter, I hurried to the forum at once.
As you can see, there is often more than one possible answer. The key thing is to choose the most likely (or most natural English) one in the context of the story.
Finally, to remember all this, I recommend one of the real Vir Drinks Beer classics:
"I thought CUM meant 'WITH'! SINCE WHEN does it have ALTHOUGHs other meanings…?!"