539. Causal clauses take either the indicative or the subjunctive, according to their construction; the idea of cause being contained, not in the mood itself, but in the form of the argument (by implication), in an antecedent of causal meaning (like proptereā), or in the connecting particles.
Quod is in origin the relative pronoun (stem quo-) used adverbially in the accusative neuter (cf. § 214.d) and gradually sinking to the position of a colorless relative conjunction (cf. English that and see § 222). Its use as a causal particle is an early special development. Quia is perhaps an accusative plural neuter of the relative stem qui-, and seems to have developed its causal sense more distinctly than quod, and at an earlier period. It is used (very rarely) as an interrogative, why? (so in classical Latin with nam only), and may, like quandō, have developed from an interrogative to a relative particle.
Quoniam (for quom iam) is also of relative origin (quom being a case form of the pronominal stem quo-). It occurs in old Latin in the sense of when (cf. quom, cum), from which the causal meaning is derived (cf. cum causal). The subjunctive with quod and quia depends on the principle of Informal Indirect Discourse (§ 592).
Quandō is probably the interrogative quam (how?) compounded with a form of the pronominal stem do- (cf. dum, dō-nec). It originally denoted time (first interrogatively, then as a relative), and thus came to signify cause. Unlike quod and quia, it is not used to state a reason in informal indirect discourse and therefore is never followed by the subjunctive.
cum tibi agam grātiās quod mē vīvere coēgistī (Att. 3.3)
when I may thank you that you have forced me to live
Cūr igitur pācem nōlō? Quia turpis est. (Phil. 7.9)
Why then do I not wish for peace? Because it is disgraceful.
Ita fit ut adsint proptereā quod officium sequuntur, taceant autem quia periculum vītant.(Rosc. Am. 1)
So it happens that they attend because they follow duty, but are silent because they seek to avoid danger.
Mihi grātulābāre quod audīssēsmē meam prīstinam dīgnitātem obtinēre. (Fam. 4.14.1)
You congratulated me because [as you said] you had heard that I had regained my former dignity.
Noctū ambulābat Themistoclēs quod somnum capere nōn posset. (Tusc. 4.44)
Themistocles used to walk about at night because [as he said] he could not sleep.
Mea māter īrāta est quia nōn redierim.(Pl. Cist. 101)
My mother is angry because I didn't return.
Note 1— Quod introduces either a fact or a statement, and accordingly takes either the indicative or the subjunctive. Quia regularly introduces a fact; hence it rarely takes the subjunctive. Quoniam (inasmuch as, since, when now, now that), has reference to motives, excuses, justifications, and the like and takes the indicative.
Note 2— Under this head what the speaker himself thought under other circumstances may have the subjunctive (§ 592.3, Note).
Ego laeta vīsa sum quia soror vēnisset. (Pl. Mil. 387)
I seemed (in my dream) glad because my sister had come.
So with quod even a verb of saying may be in the subjunctive.
Rediit quodsē oblītum nesciō quid dīceret. (Off. 1.40)
He returned because he said he had forgotten something.
Note 3— Nōn quod, nōn quia, nōn quō, introducing a reason expressly to deny it, take the Subjunctive; but the indicative sometimes occurs when the statement is in itself true, though not the true reason. In the negative, nōn quīn (with the Subjunctive) may be used in nearly the same sense as nōn quod nōn. After a comparative, quam quō or quam quodis used.
Pugilēs ingemēscunt, nōn quod doleant, sed quia profundendā vōce omne corpus intenditur. (Tusc. 2.56)
Boxers groan, not because they are in pain, but because by giving vent to the voice the whole body is put in a state of tension.
nōn quia rēctior ad Alpīs via esset, sed crēdēns (Liv. 21.31.2)
not because the route to the Alps was more direct, but believing, etc.
nōn quīn parī virtūte et voluntāte aliī fuerint, sed tantam causam nōn habuērunt (Phil. 7.6)
not that there were not others of equal courage and goodwill, but they had not so strong a reason
haec amōre magis impulsus scrībenda ad tē putāvī, quam quō tē arbitrārermonitīs et praeceptīs egēre (Fam. 10.3.4)
This I thought I ought to write to you, rather from the impulse of (prompted by) affection than because I thought that you needed advice and suggestion.
a. Quoniam and quandō (since) introduce a reason given on the authority of the writer or speaker, and take the indicative.
Locus est ā mē, quoniam ita Murēna voluit, retrāctandus. (Mur. 54)
I must review the point, since Murena has so wished.
Quandō ita vīs, dī bene vortant. (Pl. Trin. 573)
Since you so wish, may the gods bless the undertaking.
quandō ad mâiōra nātī sumus (Fin. 5.21)
since we are born for greater things
Note— The subjunctive with quoniam is unclassical.Quandō (since) in the causal sense, is mostly archaic or late. Quandō (when) is used as interrogative, relative, and indefinite.
b. Causal clauses introduced by quod, quia, quoniam, and quandō take the subjunctive in indirect discourse, like any other dependent clause (see § 580).
c. A relative, when used to express cause, regularly takes the subjunctive (see § 535.e).
d. Cum causal takes the subjunctive (see § 549).
For Substantive Clauses with quod, see § 572.