Discipulus dicit se volumen suum in bibliotheca reliquisse.

Because after dicit (he says), Latin often uses indirect statement rather than a clause with that.

So instead of saying:

• The student says that he left his book...

Latin says, literally:

• The student says himself to have left his book...

That pattern is called the accusative-and-infinitive construction:

• se = the subject of the reported statement, in the accusative
• reliquisse = the infinitive verb of the reported statement

So Discipulus dicit se volumen suum in bibliotheca reliquisse is a standard indirect statement.

Se is the reflexive accusative pronoun, and it refers back to the subject of the main verb, here discipulus.

So:

• Discipulus dicit se... reliquisse = The student says that he himself left...

If Latin used eum instead, that would normally suggest some other male person, not the student himself.

So se is used because the person doing the saying and the person who left the book are the same person.

Suum is the reflexive possessive adjective, meaning his own, her own, or their own, depending on context.

Here it refers back to se, and therefore back to discipulus:

• volumen suum = his own book/scroll

This is different from eius, which would usually mean his/her referring to someone else.

So Latin is making it clear that the book belonged to the student, not to another person.

In an indirect statement, the subject of the reported action goes into the accusative case.

In the reported idea:

• he left his book in the library

the word he becomes se in Latin because it is the subject of the infinitive reliquisse, and in this construction that subject must be accusative.

So even though se is logically the subject of reliquisse, grammatically it appears in the accusative.

Reliquisse is the perfect active infinitive of relinquo.

It shows action that happened before the action of the main verb:

• dicit = he says now
• reliquisse = to have left / that he left earlier

So the meaning is:

• The student says that he left his book in the library

not:

• The student says that he is leaving
• The student says that he leaves in a general sense

The perfect infinitive marks the leaving as already completed.

Reliquisse comes from relinquo, relinquere, reliqui, relictum = leave behind.

Its form here is:

• perfect active infinitive

You can think of it as built from the perfect stem:

• reliqu-
  • -isse

So:

• reliquisse = to have left

This is a very common form in indirect statement after verbs like dicit, putat, scit, audit, and so on.

Because in with the ablative usually means in or inside a place.

So:

• in bibliotheca = in the library

By contrast, in with the accusative usually implies motion into something:

• in bibliothecam = into the library

Here the sentence is about where the book was left, not movement into the library, so the ablative is used.

Originally, volumen means a roll or scroll, from the idea of something rolled up. In many teaching contexts it is translated simply as book.

So:

• volumen suum can be understood as his book or his scroll, depending on context

If your course translates it as book, that is perfectly normal for learners.

Yes, Latin word order is fairly flexible because the endings show the grammar.

This sentence is arranged as:

• Discipulus = subject
• dicit = main verb
• se ... reliquisse = indirect statement
• volumen suum = object inside that indirect statement
• in bibliotheca = place

But other orders are possible, for example:

• Discipulus se volumen suum in bibliotheca reliquisse dicit
• Se volumen suum in bibliotheca reliquisse discipulus dicit

These still mean essentially the same thing, though the emphasis may shift. The given order is clear and natural.

Normally, in this kind of sentence, se is needed because the indirect statement needs a subject.

So:

• dicit se reliquisse = he says that he left

If you removed se, the sentence would be incomplete or much less clear, because reliquisse still needs someone as its subject in the indirect statement.

Latin often omits subject pronouns with finite verbs, but this is different: se is not just an optional pronoun here. It is part of the accusative-and-infinitive structure.

Grammatically, suum agrees with volumen, because it is an adjective modifying it.

So it matches volumen in:

• gender: neuter
• number: singular
• case: accusative

That is why it is suum, not some other form.

However, in meaning, suum refers back to discipulus through se. So it agrees grammatically with volumen, but it points back logically to the student.

Yes. A very literal translation would be:

• The student says himself to have left his book in the library.

That sounds unnatural in English, but it shows the Latin structure clearly:

• Discipulus = the student
• dicit = says
• se = himself
• volumen suum = his book
• in bibliotheca = in the library
• reliquisse = to have left

Natural English usually changes this to:

• The student says that he left his book in the library.

Because se is reflexive: it normally points back to the subject of its own clause or governing statement.

Here the governing verb is dicit, whose subject is discipulus. So se naturally refers to discipulus.

That is one of the main jobs of se in Latin: to show that the person in the reported statement is the same as the main subject.

If Latin wanted to refer to another person, it would usually use a different pronoun, such as eum.