I quote this cautionary tale about the dangers of studying Scholastic logic in full because it's just too good not to, but my question is only about the part in bold face:

Parisius accidit, quod quidam discipulus post mortem magistro suo de die apparuit, qui indutus videbatur cappa ex pergameno, minutis litteris conscripta. Cumque magister Sella, sic enim magister vocabatur, a discipulo quereret, quid cappa illa et littere sibi vellent, respondit: "quelibet harum litterarum magis me gravat pondere suo, quam si turrem hujus ecclesiæ super collum portarem," ostensa sibi ecclesia Sancta Germani Parisiensis, in cujus prato discipulus ejus apparuit illi. "Hec," inquit, "littere sunt sophysmata et curiositates, in quibus dies meos consumpsi," et addidit: "Non possem tibi exprimere, quanto ardore crucior sub hac cappa, sed per unam guttam sudoris aliquo modo possem tibi ostendere." Cumque magister extenderet palmam, ut sudoris exciperet guttam, perforata est manus ejus a fervente gutta velut acutissima sagitta. Mox ille magister scolas logice reliquit et ad ordinem Cystertiensium se transferens ait:

Linquo "coax" ranis, "cra" corvis, vanaque vanis;
ad logicam pergo, que mortis non timet "ergo".

From a discipulo, it appears to me that the student asked Sella what the cape and its letters meant to him (Sella?), and Sella responded with the story about how the letters weigh more heavily on him than if he were carrying the church tower on his neck. From context, though, that's obviously wrong: obviously Sella asked the student why the student is wearing the cape, and the student answered with his tale of woe.

On the other hand, quæreret is active, and maybe I'm misunderstanding vellent. How does the grammar clearly indicate who asked whom, and who answered? How can you tell who is the subject of respondit?

• I think what you're missing is that quaerere a means "ask of". The cum clause has Sella as subject and quaereret as verb.
– TKR
Jun 21 '17 at 1:49
• @TKR It appears that the main lesson here for me is to stop using ab aliquo before the verb as a cheap clue that the verb is passive, and practice some more with subjunctive tenses. ;) Jun 24 '17 at 19:50

TKR has it exactly right:

Cumque magister Sella, sic enim magister vocabatur, a discipulo quereret, quid cappa illa et littere sibi vellent, respondit....

Translation:

And when Master Sella (that was the master's name) asked of the student, what the cape and letters meant, he [the student] responded....

Two idioms are used here:

• quaerere a(b) aliquo: means "to ask of someone" or "question." See meaning II.B.3 of L&S s.v. quaero. You could just as easily say ex or de.
• sibi velle means "to mean." See I.E.1.d or I.E.4. My Latin professor would say, "quid sibi vult X?" = "what does X mean?"
• In addition to sorting this out, thanks also for pointing me to these other senses of quaero and volo. Especially, I had misunderstood the antecedent of sibi as the student rather than the cape and letters. Jun 24 '17 at 20:06