Discipulus librum amissum in bibliotheca quaerit.

Because discipulus is the subject of the sentence, and here it is in the nominative singular.

Latin often marks a noun’s job in the sentence by its ending. In this sentence:

• discipulus = the student
• it is the one doing the action
• so it appears in the nominative case

For a second-declension masculine noun like discipulus, the nominative singular often ends in -us.

Because librum is the direct object of quaerit. It is the thing being searched for.

The dictionary form is liber = book, but when it is a direct object, it goes into the accusative singular:

• liber = nominative singular
• librum = accusative singular

So discipulus librum quaerit means the student is looking for the book.

Because amissum describes librum, and adjectives in Latin must agree with the noun they describe in:

• gender
• number
• case

Here, librum is:

• masculine
• singular
• accusative

So amissus must also become masculine singular accusative:

• amissus = nominative masculine singular
• amissum = accusative masculine singular

So librum amissum means the lost book.

It is the perfect passive participle of amitto, meaning lost or having been lost.

In this sentence, though, you can treat it simply as an adjective describing librum:

• liber amissus = a lost book
• librum amissum = a lost book, as direct object

English speakers often do not need to worry too much at first about the full participle analysis here. The most useful point is that it behaves like an adjective and agrees with librum.

Because in takes different cases depending on the meaning:

• in + ablative = in / on a place, showing location
• in + accusative = into / onto a place, showing motion toward

Here, in bibliotheca means in the library, so it gives the place where the searching happens. That is why bibliotheca is in the ablative singular.

Compare:

• in bibliotheca = in the library
• in bibliothecam = into the library

Most naturally, it tells you where the searching is happening: the student is searching in the library.

So the phrase usually goes with the verb quaerit rather than directly with librum amissum.

That said, Latin word order can sometimes leave room for slight ambiguity if there is no context. But in an ordinary reading, in bibliotheca is understood as the location of the action.

Quaerit is:

• present tense
• active voice
• indicative mood
• third person singular

It comes from quaero, quaerere, meaning to seek, to look for, or to search for.

So quaerit means:

• he/she searches for
• he/she is looking for

Because the subject is discipulus = student, we translate it as the student searches for or is looking for.

Classical Latin does not have articles like English the and a/an.

So:

• discipulus can mean the student or a student
• librum can mean the book or a book

Which one sounds best depends on context and on the translation you have been given. Latin leaves that distinction unstated unless the context makes it clear.

Latin word order is more flexible than English word order because the noun endings show each word’s role.

So all of these could express roughly the same basic idea:

• Discipulus librum amissum in bibliotheca quaerit.
• Discipulus in bibliotheca librum amissum quaerit.
• Librum amissum discipulus in bibliotheca quaerit.

The differences are usually about emphasis, not basic meaning.

In this sentence, putting quaerit at the end is very normal Latin style. The main structure is still easy to see because:

• discipulus = subject
• librum amissum = object
• in bibliotheca = location
• quaerit = verb

You can tell because librum and amissum agree with each other:

• both are masculine
• both are singular
• both are accusative

That matching is a strong clue that amissum is describing librum.

So you should read them together as:

• librum amissum = the lost book / a lost book

This is very common in Latin: agreement helps you see which words belong together, even when the word order is flexible.