Discipula negat se stilum in mensa reliquisse et dicit fratrem eum cepisse.

Latin often uses the accusative-and-infinitive construction for reported speech or thought instead of a separate word like that.

So:

• negat se stilum in mensa reliquisse = she denies that she left the pen on the table
• dicit fratrem eum cepisse = she says that her brother took it

In this construction:

• the subject of the reported clause goes into the accusative
• the verb of the reported clause becomes an infinitive

That is why you see se ... reliquisse and fratrem ... cepisse.

Se is the reflexive pronoun. It refers back to the subject of the main verb, here discipula.

So negat se reliquisse means the schoolgirl denies that she herself left it.

If Latin used eam, that would normally mean her in the sense of some other female person, not the subject discipula.

So:

• se = herself, referring back to the main subject
• eam = her, referring to another female

Because fratrem is the subject of an infinitive in an indirect statement.

After dicit, Latin does not say something like frater cepit inside the reported clause. Instead it uses:

• fratrem as the accusative subject
• cepisse as the infinitive verb

So dicit fratrem eum cepisse literally works like she says her brother to have taken it, which is how Latin expresses she says that her brother took it.

If it were a direct statement, you would expect frater cepit.

Reliquisse is the perfect active infinitive of relinquere.

Its basic parts are:

• relinquo = I leave
• reliqui = I left / have left
• relictum = left

From the perfect stem reliqu-, Latin forms the perfect infinitive reliquisse.

In indirect statement, the perfect infinitive usually shows action that happened before the main verb. So here the leaving happened before the denying.

Because the clause after negat is an indirect statement.

In an indirect statement, Latin normally does not use a finite verb such as reliquit. It uses an infinitive instead.

So Latin says:

• negat se reliquisse

not:

• negat se reliquit

The infinitive is required by the construction.

Cepisse is the perfect active infinitive of capere.

It is used for the same reason that reliquisse is used: the clause after dicit is also an indirect statement.

So:

• dicit fratrem eum cepisse = she says that her brother took it

The perfect infinitive shows that the taking is viewed as already completed relative to dicit.

Eum refers back to stilum.

Even though English says it, Latin has to choose a gendered pronoun that matches the noun it refers to. Since stilus is masculine, the pronoun is masculine too:

• stilus = masculine
• eum = masculine accusative singular

It is in the accusative because it is the direct object of cepisse: the brother took it.

Because in takes:

• the ablative for location
• the accusative for motion toward

So:

• in mensa = on the table / in the table area, showing where the pen was
• in mensam = onto the table, showing movement toward it

Here the idea is location, not motion, so Latin uses the ablative mensa.

Latin often leaves out possessive words like suus, sua, suum when the meaning is already clear from context.

So fratrem can naturally be understood as her brother here, because the subject is discipula and the context makes the relationship obvious.

If the writer wanted to be more explicit or emphatic, they could say fratrem suum.

Latin word order is much freer than English word order because Latin endings show how words function in the sentence.

For example:

• discipula is nominative, so it is the subject
• se, stilum, fratrem, and eum are accusative, so they have other roles
• reliquisse and cepisse are infinitives

Because the endings already tell you the grammar, Latin can move words around for emphasis or style. Verbs also often come near the end.

So this sentence may feel unusual in English order, but its structure is perfectly normal Latin.