a. The underlying principle is one already familiar to you (cf.  Sec. 389.a).  When the cum\ clause states a fact and simply _fixes the time_ at which the main action took place, the indicative mood is used.  So, in the first example, cum in Gallia eram\ fixes the time when I saw Caesar.

b. On the other hand, when the cum\ clause _describes the circumstances_ under which the main act took place, the subjunctive mood is used.  So, in the second example, the principal clause states that Caesar made an attack, and the cum\ clause describes the circumstances under which this act occurred.  The idea of time is also present, but it is subordinate to the idea of description.  Sometimes the descriptive clause is one of cause and we translate cum\ by _since_; sometimes it denotes _concession_ and cum\ is translated although.

396. RULE.  Constructions with Cum. The conjunction /cum\ means /when\, /since\, or /although\.  It is followed by the subjunctive unless it means /when\ and its clause fixes the time at which the main action took place.

NOTE. Cum\ in clauses of description with the subjunctive is much more common than its use with the indicative.

397. Note the following sentences: 

  1.  Oppidum erat parvum magnitudine sed magnum multitudine hominum,
    the town was small in size but great in population.

  2.  Homo erat corpore infirmus sed validus animo,
       the man was weak in body but strong in courage.

a. Observe that magnitudine\, multitudine\, corpore\, and animo\ tell in what respect something is true.  The relation is one covered by the ablative case, and the construction is called the ablative of specification.

398. RULE.  Ablative of Specification. The ablative is used to denote /in what respect\ something is true.

399. IDIOMS

  aliquem certiorem facere, to inform some one (lit. to make some
    one more certain)
  certior fieri, to be informed (lit. to be made more certain)
  iter dare, to give a right of way, allow to pass
  obsides inter se dare, to give hostages to each other

400. EXERCISES

I. 1.  Helvetii cum patrum nostrorum tempore domo prefecti essent, consulis exercitum in fugam dederant. 2.  Cum Caesar in Galliam venit, Helvetii alios agros petebant. 3.  Caesar cum in citeriore Gallia esset, tamen de Helvetiorum consiliis certior fiebat. 4.  Cum Helvetii bello clarissimi essent, Caesar iter per provinciam dare recusavit. 5.  Legatus cum haec audivisset, Caesarem certiorem fecit. 6.  Cum principes inter se obsides darent, Romani bellum paraverunt. 7.  Caesar, cum id nuntiatum esset, maturat ab urbe proficisci. 8.  Ne virtute quidem Galli erant pares Germanis. 9.  Caesar neque corpore neque animo infirmus erat. 10.  Illud bellum tum incepit cum Caesar fuit consul.