Quaero librum qui mihi plura exempla de magistratibus praebeat.

Because librum is the direct object of quaero.

• quaero = I seek / I am looking for
• The thing being sought is the book
• In Latin, a direct object normally takes the accusative case

So:

• liber = book (nominative, dictionary form)
• librum = book (accusative singular, used as the object)

In this sentence, Quaero librum means I am looking for a book.

Because qui is a relative pronoun introducing a relative clause.

The clause is:

• qui mihi plura exempla de magistratibus praebeat

This describes librum, so qui means:

• who
• which
• that

depending on what sounds natural in English.

Since librum is a thing, English usually translates it as that or which:

• a book that provides...
• a book which provides...

Latin uses the same relative pronoun system for all of these ideas.

Because the case of the relative pronoun depends on its role inside its own clause, not on the case of the noun it refers to.

Here, qui refers back to librum, but within the relative clause it is the subject of praebeat.

So:

• librum is accusative because it is the object of quaero
• qui is nominative because it is the subject of praebeat

A useful rule:

• the relative pronoun gets its gender and number from its antecedent
• but its case from its function in the relative clause

So qui is:

• masculine singular, matching librum
• nominative, because it is the subject of praebeat

This is one of the most important things to notice in the sentence.

The subjunctive praebeat is used because the speaker is not talking about a specific known book, but about the kind of book they are looking for. This is often called a relative clause of characteristic.

So the idea is not:

• I am looking for the book, which provides...
  (a specific book already known)

but more like:

• I am looking for a book that would provide...
• I am looking for a book that might provide...
• I am looking for a book that provides...
  (in the sense of the sort of book that provides)

The subjunctive gives the sense of:

• possibility
• desired characteristic
• an indefinite or not-yet-identified thing

So praebeat suggests a book of the kind that would provide me with more examples.

Yes, many learners notice that it feels a bit like purpose: I am looking for a book in order that it may provide me...

But the standard explanation is usually relative clause of characteristic, because the clause describes the type of book being sought rather than stating a straightforward purpose clause.

You can think of the difference like this:

• purpose focuses on the goal
• characteristic focuses on the kind of thing being described

Here the sentence means something like:

• I seek a book of a kind that would provide me with more examples about magistrates.

So although there is a purpose-like flavor, relative clause of characteristic is the usual label.

Because praebeo commonly takes:

• a direct object = the thing provided
• a dative = the person for whom or to whom it is provided

So in:

• mihi plura exempla praebeat

the parts are:

• mihi = to me / for me (dative)
• plura exempla = more examples (accusative direct object)

So the phrase means:

• provide me with more examples
• literally, provide more examples to/for me

This is very common with verbs of giving, showing, offering, and providing.

Because plura is the neuter plural comparative adjective/adjective-like form meaning more, and it agrees directly with exempla.

So:

• exemplum = example
• exempla = examples
• plura exempla = more examples

This is often the most natural way to say more + plural noun in Latin.

You may also see plus used, but plus is more like a neuter singular substantive meaning more as a quantity. With countable plural nouns, plures/plura is often the better fit.

Since exempla is neuter plural, the form is:

• plura

not plures.

Because it agrees with exempla, and exempla is neuter plural.

Remember:

• exemplum is a neuter noun
• its plural nominative/accusative form is exempla

Any adjective modifying it must match in:

• gender
• number
• case

So:

• plura = neuter plural accusative
• exempla = neuter plural accusative

Together they form the direct object of praebeat.

Magistratibus is ablative plural because it follows the preposition de.

• de
  • ablative = about, concerning, on

So:

• de magistratibus = about magistrates

This is a very common Latin prepositional phrase pattern. Whenever you see de, expect the noun after it to be in the ablative.

It most naturally goes with exempla:

• plura exempla de magistratibus = more examples about magistrates

So the whole relative clause means:

• that may provide me with more examples about magistrates

In other words, the book provides examples, and those examples concern magistrates.

Here it means I look for / seek.

Latin quaero can have several related meanings, including:

• seek
• look for
• ask
• inquire

The context tells you which one is meant.

Since the object is librum (a book), the natural meaning is:

• I am looking for a book
• I seek a book

If it meant ask, you would expect a different kind of object or construction.

Because Latin has no definite or indefinite article like English the or a/an.

So librum can mean:

• a book
• the book

The context tells you which is intended.

In this sentence, because the speaker is searching for some suitable book and the relative clause uses the subjunctive, the sense is clearly indefinite:

• a book

not the book.

Yes. Quaero is first person singular present active indicative:

• quaero = I seek / I look for / I am looking for

In English, the best translation depends on style:

• I am looking for a book...
• I seek a book...

Both are fine.
In beginner-friendly English, I am looking for often sounds the most natural.

A helpful way to break it down is:

• Quaero librum = I am looking for a book
• qui... praebeat = that may / would provide...

Inside the relative clause:

• qui = subject
• mihi = indirect object
• plura exempla = direct object
• de magistratibus = prepositional phrase modifying exempla
• praebeat = verb

So the structure is:

• main clause: Quaero librum
• relative clause describing librum: qui mihi plura exempla de magistratibus praebeat

A very natural way to understand it is:

• I’m looking for a book that would give me more examples about magistrates.
• I’m looking for a book that can provide me with more examples about magistrates.
• I’m looking for a book that provides me with more examples about magistrates.

The important nuance is that the book is not specifically identified yet. It is the kind of book the speaker wants to find. That is exactly why Latin uses praebeat rather than praebet here.