Magister ostendit quid error sit et quomodo discipuli eum corrigere debeant.

It has one main clause and two dependent clauses:

• Main clause: Magister ostendit
• First dependent clause: quid error sit
• Second dependent clause: quomodo discipuli eum corrigere debeant

The two dependent clauses are joined by et. Both of them depend on ostendit and explain what the teacher shows.

So the pattern is basically:

• The teacher shows
  • what the error is
  • and how the students should correct it

Because both dependent clauses are indirect questions.

In Latin, when a question is reported rather than asked directly, the verb normally goes into the subjunctive.

So:

• quid error sit = an indirect question introduced by quid (what)
• quomodo discipuli eum corrigere debeant = an indirect question introduced by quomodo (how)

This is a very common Latin pattern after verbs like ask, know, show, explain, see, and similar verbs.

Quid is the interrogative pronoun meaning what. It introduces the indirect question quid error sit.

Here it is not a relative word like which or that. It is asking for identification: what the error is.

A learner often wonders why it is quid and not something else. The reason is that Latin uses the interrogative pronoun quis/quid for who?/what? questions, including indirect ones.

Because this is an interrogative clause, not a relative clause.

• quid = what?
• quod often means which/that as a relative pronoun, or because/that as a conjunction

In this sentence, the teacher is showing what the error is, so Latin uses the interrogative form quid.

Because error is the subject of sit.

In quid error sit, the verb is a form of esse (to be), and error is the thing being identified. So Latin keeps it in the nominative:

• error = the error as subject
• sit = is

So the clause means what the error is, not what the error as a direct object would be.

Quomodo means how or in what way.

It introduces the second indirect question: quomodo discipuli eum corrigere debeant.

So this clause is asking about the method or manner of correction, not the purpose. In other words, it means how the students should correct it, not so that the students may correct it.

Because discipuli is the subject of debeant.

• discipuli = nominative plural
• debeant = 3rd person plural subjunctive

They match each other:

• discipuli ... debeant = the students should / ought to ...

If discipuli were the object, you would expect discipulos instead.

Eum refers back to error.

That makes sense because the students are supposed to correct it, and it = the error.

It is:

• masculine singular because error is masculine singular
• accusative because it is the direct object of corrigere

So:

• error = the error
• eum = it / him, referring here to the error

Because it depends on debeant.

Latin commonly uses debeo + infinitive to mean ought to, should, or must.

So:

• corrigere debeant = they should correct
• literally: they ought to correct

This is the same basic idea as English should correct, where the second verb stays in a non-finite form.

Formally, ostendit can be either:

• present: he shows
• perfect: he showed / has shown

The form is ambiguous by itself. Context tells you which meaning is intended.

In a sentence like this, the present is usually the most natural reading, because it sounds like a general statement or explanation. But in a narrative context, perfect would also be possible.

Not as much as it would be in English.

Latin relies heavily on endings, so the sentence can be more flexible in word order. This order is perfectly natural, but other arrangements could also work if the writer wanted a different emphasis.

For example, the important grammatical relationships are clear from the forms:

• magister = subject of ostendit
• error = subject of sit
• discipuli = subject of debeant
• eum = object of corrigere

So the word order helps style and emphasis, but the endings carry most of the grammar.