Magister librum apud se retinet, donec discipulus mendum corrigat.

Because librum is the direct object of retinet.

• retinet = keeps / retains / holds back
• The thing being kept is the book
• In Latin, the direct object usually goes in the accusative case

So:

• magister = the teacher (subject, nominative)
• librum = the book (direct object, accusative)

Apud se means something like with himself, by him, or in his possession.

In this sentence, it does not mainly mean physical companionship, but rather that the teacher keeps the book with him / in his own custody.

Breakdown:

• apud
  • accusative often means at the house of, among, with, or in the possession of
• se is the reflexive pronoun, here meaning himself

So librum apud se retinet suggests:

• the teacher keeps the book with himself
• the book remains in the teacher’s hands

Because se is the reflexive pronoun, and it refers back to the subject of its own clause, which is magister.

In the main clause:

• Magister ... retinet = The teacher keeps ...
• apud se = with himself

If Latin used eum, that would normally refer to some other male person, not back to the teacher himself.

So:

• se = himself, herself, themselves, when referring back to the subject of the clause
• eum = him, referring to someone else

Because donec can take the subjunctive when the action is viewed as anticipated, expected, or still in the future from the perspective of the main clause.

Here the idea is:

• the teacher keeps the book
• until the student corrects the mistake

The correction has not yet happened at the moment described by retinet. It is something awaited.

So donec discipulus mendum corrigat means roughly:

• until the student should corrects the mistake
• more naturally in English: until the student corrects the mistake

Latin often uses the subjunctive after donec in this kind of prospective clause.

Corrigit would be indicative, while corrigat is subjunctive.

The subjunctive here shows that the correcting is:

• not yet realized at the time of the main action
• looked forward to as the event that will end the teacher’s keeping the book

So the sentence presents the correction as the expected endpoint.

Very roughly:

• donec ... corrigit would sound more like a straightforward factual limit
• donec ... corrigat gives a more prospective sense: until he correct it

In many classroom examples, this is exactly why the subjunctive is chosen.

Because the main verb is in a primary tense:

• retinet = present

In the usual sequence of tenses, a present main verb is followed by a present or perfect subjunctive, depending on the time relationship.

Here the subordinate action is contemporaneous/future relative to the main verb:

• the teacher is keeping the book now
• the student’s correction is the awaited event that will end that situation

So the present subjunctive corrigat is the natural choice.

Mendum is accusative singular neuter.

It is the direct object of corrigat:

• discipulus mendum corrigat = the student corrects the mistake

Its nominative and accusative singular are both mendum, because it is a second-declension neuter noun.

So:

• nominative singular: mendum
• accusative singular: mendum

The form looks the same in both cases, which is normal for neuter nouns.

Tenet means holds or has.

Retinet is stronger: it means keeps back, retains, holds onto, or does not return.

In this sentence, retinet suggests that the teacher is deliberately keeping the book until something happens. That fits the context much better than a simple tenet.

So the sense is not just:

• the teacher has the book

but rather:

• the teacher keeps the book back / keeps it in his possession

Here donec means until.

It introduces the point at which the action of the main clause will stop:

• the teacher keeps the book
• until the student corrects the mistake

Depending on context, donec can sometimes mean things like while or as long as, but in this sentence until is clearly the right sense.

Because there are two clauses, and each clause has its own subject.

1. Main clause:

   • Magister librum apud se retinet
   • subject: magister

2. Subordinate clause:

   • donec discipulus mendum corrigat
   • subject: discipulus

So both nouns are nominative because each is the subject of its own verb:

• magister goes with retinet
• discipulus goes with corrigat

Yes, but it is normal Latin-style flexibility rather than something strange.

The sentence is:

• Magister librum apud se retinet, donec discipulus mendum corrigat.

Some points:

• magister comes first as the topic or subject
• librum appears early, before the verb, which is very common
• apud se sits near retinet, helping describe how the teacher keeps the book
• the verb retinet comes at the end of its clause, which is a very typical Latin pattern
• then the donec clause follows

Latin could rearrange this in other ways without changing the basic meaning, for example:

• Magister apud se librum retinet, donec discipulus mendum corrigat.

The original order is simply a natural and clear one.

Not without changing the nuance.

• secum = with himself, often in the sense of accompaniment
• apud se = with him / in his possession / at his place

In this sentence, the idea is possession or custody, so apud se is better.

Compare the feel:

• secum might suggest carrying something along with oneself
• apud se retinet suggests keeping something in one’s own hands or keeping hold of it

So apud se is the more idiomatic choice here.

No. Donec can take either the indicative or the subjunctive, depending on the meaning and how the writer presents the action.

A simple rule of thumb:

• indicative: the limit is presented more as a fact
• subjunctive: the limit is presented as anticipated, intended, or still pending

In this sentence, the correction is the awaited condition for returning the book, so the subjunctive corrigat is very natural.

Because Latin normally expresses until with a full clause, not with an infinitive.

English can say things like:

• He kept the book until correcting the mistake
• though even in English that sounds less natural than until the student corrected the mistake

Latin strongly prefers a clause with a finite verb after donec:

• donec discipulus mendum corrigat

That is the standard Latin way to express the idea.

Not for the grammar itself.

Ancient Latin manuscripts did not use punctuation the way modern printed texts do. The comma here is a modern editorial aid to show the boundary between:

• the main clause
• the donec clause

So it helps the reader, but it is not something the grammar depends on.