[1881.] A protasis with cum is often followed by an emphatic apodosis introduced by tum.
The protasis denotes what is general or common or old; the apodosis what is special or strange or new. In classical Latin tum is often emphasized by maximē, in prīmīs, vērō, &c.
In this use the mood is more commonly the indicative and the time of the two verbs is apt to be identical: as, quom mihi paveō, tum Antiphō mē excruciat animī, T. Ph. 187, whilst for myself I tremble, Antipho puts me in a perfect agony of soul. But cum anteā distinēbar maximīs occupātiōnibus, tum hōc tempore multō distineor vehementius, Fam. 12, 30, 2, I was distracted by most important engagements before, but now I am very much more distracted. Less frequently the subjunctive, to denote cause or concession ([1877]): as, cum tē ā pueritiā tuā dīlēxerim, tum hōc multō ācrius dīligō, Fam. 15, 9, 1, whereas I have always loved you from your boyhood, for this I love you with a far intenser love. By abridgement of the sentence ([1057]), cum . . . tum come to be copulative conjunctions ([1687]): as, mōvit patrēs cōnscrīptōs cum causa tum auctor, L. 9, 10, 1, both the cause and its supporter touched the conscript fathers.
[quoniam.]
[1882.] quoniam, compounded of quom and iam, when now, refers primarily to time, but is seldom so used and only by early writers. The temporal meaning passed early into an exclusively causal meaning, since. In both meanings it regularly introduces the indicative ([1721]). For special reasons, however, the subjunctive is used, as in indirect discourse ([1725]), or by attraction ([1728]).
[1883.] (1.) quoniam, when now, used of time in early Latin, has sometimes as a correlative continuō, subitō, or extemplō; it usually introduces the present indicative ([1590]): as,
is quoniam moritur, numquam indicāre id fīliō voluit suō, Pl. Aul. 9, when he was on his dying bed, he ne’er would point it out to his own son, of a hidden treasure. quoniam sentiō quae rēs gererētur, nāvem extemplō statuimus, Pl. B. 290, when now I saw what was doing, we stopped the ship at once.
[1884.] (2.) quoniam, since, seeing that, now that, with the indicative, introduces a reason, usually one known to the person addressed, or one generally known: as,
vēra dīcō, sed nēquīquam, quoniam nōn vīs crēdere, Pl. Am. 835, the truth I speak, but all in vain, since thou wilt not believe. vōs, Quirītēs, quoniam iam nox est, in vestra tecta discēdite, C. 3, 29, do you, citizens, since it is now grown dark, depart and go to your own several homes. quoniam in eam ratiōnem vītae nōs fortūna dēdūxit, ut sempiternus sermō dē nōbīs futūrus sit, caveāmus, QFr. 1, 1, 38, since fortune has set us in such a walk of life that we are to be eternally talked about, let us be on our guard. Often in transition: as, quoniam dē genere bellī dīxī, nunc dē magnitūdine pauca dīcam, IP. 20, since I have finished speaking about the character of the war, I will now speak briefly about its extent. With the subjunctive in indirect discourse ([1725]): as, crēbrīs Pompēī litterīs castīgābantur, quoniam prīmō venientem Caesarem nōn prohibuissent, Caes. C. 3, 25, 3, they were rebuked in numerous letters of Pompey, ‘because they had not kept Caesar off as soon as he came.’