Here are somethings from Story of St Nicholas of The Golden Legend (Legenda Aurea), by Jacobus de Voragine (1228-1298)
[1] Hic prima die, dum (balnearetur), erectus stetit in pelvi = On the first day as he was being bathed, he stood up in the basin
BALBEARETUR = was being bathed? I can not look up it in the William Whitaker dictionary, is it active or passive? Additionally, where I can find a dictionary of Mendieval Latin?
[2] (Factus autem iuvenis) aliorum devitans lascivias ecclesiarum potius terebat limina et quidquid ibi de sacra scriptura intellegere poterat, memoriter retinebat = When he was young, avoiding the wantonness of the others, he visited churches and was remembered all the selections form holy scripture that he was able to understand there
Factus autem iuvenis = when youth is happened ?
The passive past participle FACTUS here is used as a predicate adjective, right?
[3] (Parentibus vero suis defunctis) cogitare coepit, qualiter tantam divitiarum copiam non ad laudem humanam, sed ad Dei gloriam dispensaret = After his parents died, he began to consider how to distribute his wealth, not to gain the praise of men, but for the glory of God
Parentibus vero suis defunctis = when his parents died
It is clearly ablative absolute, but what is the function of VERO here?
[1] ‘balnearetur’ is passive, maybe because the child “is bathed/washed” by another person.
As for Medieval Latin dictionaries, you can use Niermeyer’s Lexicon or Du Cange’s Glossarium. Forcellini can also be useful.
[2] Yes, ‘factus’ is the past participle, litterally “having been made a young man” > “having become a young man”.
In your translation “was remembered” (passive) should be “remembered” (‘retinebat’ is active).
[3] ‘vero’ doesn’t really mean much here and it could be left untranslated. In theory, it is supposed to mark a slight opposition, ‘but’. Here the story shifts from his life when his parents were alive to a completely different situation where his parents are dead. This could explain the use of ‘vero’.
No, balnearetur is the verb balneo, balneare in the passive imperfective subjunctive. An easy way to remember how the passive imperfective subjunctive is formed is to take it as the present active infinitive to which are added the passive personal endings (-r, -ris, -tur, -mur, -mini, -ntur).