Puer respondet se stilum sub mensa reliquisse.

The sentence has two main parts:

• Puer respondet = The boy answers / replies
• se stilum sub mensa reliquisse = what he answers, in the form of an indirect statement

So the pattern is:

• main verb of saying/reporting
  • accusative subject
    • infinitive

That is a very common Latin construction. Instead of using that + a finite verb, Latin often uses an accusative-and-infinitive construction.

Se is the accusative form of the reflexive pronoun, meaning himself / herself / itself / themselves, depending on context.

Here it refers back to the subject of the main clause:

• Puer respondet = The boy replies
• se ... reliquisse = that he had left ...

So se means himself, but in natural English we usually translate it as just he in indirect statement:

• The boy replies that he left / had left the pen under the table.

Latin uses se because the subject of the reported statement is the same person as the subject of respondet.

Because Latin uses a reflexive pronoun when the person referred to is the same as the subject of the main clause.

Compare:

• Puer respondet se stilum reliquisse. = The boy replies that he himself left the pen. = the one who replied is also the one who left it.

But:

• Puer respondet eum stilum reliquisse. = The boy replies that he (some other male person) left the pen.

So:

• se = refers back to the main subject
• eum = refers to someone else

Because after verbs of saying, thinking, knowing, perceiving, and similar verbs, Latin often uses an indirect statement.

In English we say:

• The boy replies that he left the pen.

Latin usually says:

• The boy replies himself the pen to-have-left.

That sounds strange in English, but grammatically it is:

• se = accusative subject of the indirect statement
• reliquisse = infinitive

So reliquisse is not standing alone randomly; it is the normal verb form for reported speech in this construction.

Reliquisse is the perfect active infinitive of relinquere (to leave behind).

The perfect infinitive usually shows action earlier than the main verb.

So:

• respondet = he replies (present)
• reliquisse = to have left / to have left earlier

That is why English often translates it as:

• The boy replies that he left the pen under the table or more explicitly
• The boy replies that he had left the pen under the table

Both can work depending on context, but the Latin form clearly marks the leaving as prior to the replying.

Because stilum is the direct object of reliquisse.

The verb relinquere takes a direct object:

• relinquere aliquid = to leave something

So:

• se = subject of the infinitive clause, in the accusative because of indirect statement
• stilum = object of reliquisse, also accusative as a normal direct object

Even though both are accusative, they do different jobs:

• se = who did the leaving
• stilum = what was left

Because sub can take either the accusative or the ablative, depending on meaning.

• sub + accusative often suggests motion toward / to a position under
• sub + ablative often suggests location, meaning under

Here the meaning is location:

• sub mensa = under the table

So the ablative is used because the pen was left in a place under the table, not moved to under the table as the focus of the phrase.

Mensa is ablative singular of mensa, mensae.

It is ablative because it follows sub in a locational sense:

• sub mensa = under the table

This is a good example of how prepositions in Latin govern specific cases, and sometimes the case changes the nuance.

Latin word order is much more flexible than English word order.

This sentence is arranged as:

• Puer = boy
• respondet = replies
• se = himself / that he
• stilum = the pen
• sub mensa = under the table
• reliquisse = to have left

A more literal order in English would be:

• The boy replies that he left the pen under the table.

Latin often places the infinitive at or near the end of the indirect statement, and that is what happens here with reliquisse.

So the word order is normal and natural Latin, even though it does not match English word order exactly.

In an accusative-and-infinitive construction, the subject of the infinitive appears in the accusative case.

So in:

• se stilum sub mensa reliquisse

the accusative pronoun se is understood as the doer of the action reliquisse.

You can think of it like this:

• se reliquisse = that he left / to have left
• stilum = the thing left

Latin does not need a nominative subject here because the subject belongs to the infinitive clause, not to a finite verb.

Yes. Respondet comes from respondere, which can mean:

• to answer
• to reply
• sometimes to respond

Which English word sounds best depends on context.

So this sentence could naturally be understood as:

• The boy answers that he left the pen under the table or
• The boy replies that he left the pen under the table

Because in this kind of indirect statement, Latin usually does not use a separate word meaning that.

English says:

• The boy replies that he left the pen under the table.

Latin instead uses the grammar of:

• accusative subject
  • infinitive

So the idea of that is built into the construction itself:

• se ... reliquisse = that he had left ...

In other words, there is no missing word; the sentence is complete exactly as it stands.

The verb is relinquere.

Its principal parts are:

• relinquo
• relinquere
• reliqui
• relictum

From the perfect stem reliqu-, Latin forms the perfect active infinitive:

• reliquisse = to have left

So if you are parsing it:

• verb: relinquere
• form: perfect active infinitive

Yes, and that can help you see the pattern more clearly.

For example:

• Pueri respondent se stilum sub mensa reliquisse.
• The boys reply that they left the pen under the table.

Notice that se can also mean themselves. The reflexive pronoun does not change form for singular vs. plural in the accusative/ablative.

So context tells you whether se means:

• himself
• herself
• itself
• themselves

Here, because puer is singular, we understand se as himself / he.