Puer librum in subsellio relinquit.

Because puer is in the nominative case, which is the case normally used for the subject in Latin.

• puer = boy
• nominative singular of puer, puerī

So puer is the boy / a boy, the one doing the action.

A native English speaker may expect word order to show the subject, but in Latin the ending is usually more important than the position in the sentence.

Because librum is in the accusative case, which is the case used for the direct object.

• liber = nominative singular, book as subject
• librum = accusative singular, book as object

In this sentence, the book is not doing the action; it is the thing being left behind. So Latin uses librum.

Here in means in/on in the sense of location, not movement toward a place.

Latin uses:

• in + ablative for location: in the seat / on the bench
• in + accusative for motion into: into the seat / onto the bench

So:

• in subsellio = in/on the seat/bench where the book is left
• in subsellium would suggest movement into/onto it

Because the sentence describes where the book ends up being left, subsellio is in the ablative.

Subsellio is ablative singular.

Its dictionary form is:

• subsellium, subselliī = bench, seat, school bench

Because it follows in with the meaning of location, it appears in the ablative: in subsellio.

Relinquit is:

• 3rd person singular
• present tense
• active voice
• indicative mood

It comes from the verb relinquō, relinquere, relīquī, relictum, meaning leave behind, abandon, or leave.

So relinquit means he leaves, she leaves, or it leaves.
Because the subject here is puer, it means the boy leaves.

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

So:

• puer can mean the boy or a boy
• librum can mean the book or a book

You understand which one is meant from context.

This is very common for English speakers to notice, because English usually requires an article, but Latin does not.

The ending -t tells you it is 3rd person singular.

Compare:

• relinquit = he/she/it leaves
• relinquunt = they leave

Since the subject is puer (boy, singular), relinquit matches it perfectly.

Yes. Latin word order is much more flexible than English word order because the case endings show the grammatical roles.

For example, these can mean the same thing:

• Puer librum in subsellio relinquit.
• Librum puer in subsellio relinquit.
• In subsellio puer librum relinquit.

The basic meaning stays the same because:

• puer is nominative
• librum is accusative
• subsellio is ablative after in

However, changing the order can change the emphasis.

Because Latin verbs often already include the subject in their endings.

The ending of relinquit already tells you he/she/it. So Latin does not need to add a separate pronoun unless it wants emphasis.

For example:

• relinquit = he/she leaves
• is relinquit would be something like he leaves, with extra emphasis on he

Since puer is already present, no pronoun is needed at all.

Here are the usual dictionary forms:

• puer, puerī — boy
• liber, librī — book
• subsellium, subselliī — bench, seat, school bench
• relinquō, relinquere, relīquī, relictum — leave behind, leave

This helps you see how the forms in the sentence were made:

• puer from puer
• librum from liber
• subsellio from subsellium
• relinquit from relinquō

In natural English, on the bench or on the seat may sound better in many contexts, even though Latin uses in.

Latin in can cover meanings that English splits between in and on, depending on the object and the situation. So in subsellio does not always have to be translated mechanically as in the bench.

A smoother English translation may be:

• The boy leaves the book on the bench.

They belong to different declensions:

• puer, puerī — 2nd declension masculine
• liber, librī — 2nd declension masculine
• subsellium, subselliī — 2nd declension neuter

This is useful because it helps explain the endings:

• puer = nominative singular
• librum = accusative singular masculine
• subsellio = ablative singular neuter

Even though all three are 2nd declension, they do not all look the same because gender and case affect the ending.

Many 2nd declension masculine nouns end in -us, like servus. But some end in -er, such as:

• puer = boy
• liber = book

These are still 2nd declension nouns, but their nominative singular is different.

A learner may expect something like puerus or liberus, but those are not the correct forms. You simply have to learn that some 2nd declension masculines use -er in the nominative.

Also, different -er nouns behave differently in the stem:

• puer, puerī keeps the e
• ager, agrī loses the e

So the dictionary form matters.