Puer matri confitetur se mendacium dixisse.

Because puer is the subject of confitetur.

In Latin, the subject of a finite verb is normally in the nominative case. Here, puer is the person doing the confessing, so nominative is exactly what we expect.

• puer = the boy
• confitetur = confesses / admits

So puer is the one who confesses.

Because confitetur can take the person confessed to in the dative case.

So:

• matri = to his mother / to the mother

This is similar to the indirect object in English:

• The boy confesses to his mother...

In Latin, many verbs of telling, showing, trusting, pleasing, and similar ideas can take a dative. Here, matri is the person receiving the confession.

Because confitetur is a deponent verb form.

The verb is confiteor, confiteri, confessus sum, meaning to confess / admit. Deponent verbs look passive in many forms, but they have an active meaning.

So:

• confitetur looks like a passive form
• but it means he confesses, not he is confessed

This is a very common thing in Latin, and learners often need time to get used to it.

This is an indirect statement (also called an accusative-and-infinitive construction).

Latin often uses this structure where English uses that + a clause.

So instead of:

• The boy confesses that he told a lie

Latin says, more literally:

• The boy confesses himself to have told a lie

The pattern is:

• se = the subject of the indirect statement, in the accusative
• dixisse = the verb of the indirect statement, in the infinitive

This construction is extremely common after verbs of saying, thinking, knowing, perceiving, and admitting.

Because se is the reflexive pronoun, and it refers back to the subject of the main clause, puer.

So here:

• puer ... confitetur se...
• the boy confesses that he himself ...

If Latin used eum, that would normally mean that he / that another male person, not necessarily the boy himself.

So se makes it clear that the person who told the lie is the same person as the one doing the confessing.

Because the lie-telling happened before the confessing.

The main verb is:

• confitetur = he confesses / is confessing

The infinitive is:

• dixisse = to have said

In indirect statement, Latin uses the tense of the infinitive to show time relative to the main verb:

• present infinitive = same time as the main verb
• perfect infinitive = earlier than the main verb
• future infinitive = later than the main verb

So se mendacium dixisse means:

• that he had told / has told a lie
• in other words, the saying of the lie happened before the confession

Because it is the direct object of dixisse.

The verb behind dixisse is dicere, meaning to say. What did he say? mendacium.

So:

• mendacium = a lie
• dixisse = to have said
• mendacium dixisse = to have said a lie / to have told a lie

Even though dixisse is an infinitive, it can still take its own object.

Because in this kind of sentence Latin usually does not use a conjunction like English that.

English says:

• He confesses that he told a lie.

Latin normally uses:

• a verb of saying/thinking/admitting
• followed by accusative + infinitive

So instead of a separate word meaning that, Latin lets the structure itself show the idea.

That is why you get:

• se mendacium dixisse

rather than a subordinate clause introduced by a word meaning that.

Yes, different word orders are possible.

Latin word order is more flexible than English because the endings show each word’s role. So the exact order can shift for emphasis or style.

For example, these would still be understandable Latin:

• Puer matri confitetur se dixisse mendacium.
• Matri puer confitetur se mendacium dixisse.

The original order is perfectly natural, though. In se mendacium dixisse, placing mendacium before dixisse is a normal way to keep the object close to its infinitive.

By itself, matri literally means to the mother.

However, Latin often leaves out possessive words like his when they are obvious from context. In a sentence like this, readers will usually understand matri as to his mother, unless the wider context suggests some other mother.

So grammatically:

• matri = to the mother

But idiomatically in context, it often means:

• to his mother

Because se already does that job.

Inside the indirect statement, Latin does not use a nominative subject such as ille or is. Instead, the subject of the infinitive is put in the accusative.

So:

• se = the subject of dixisse
• dixisse = infinitive verb

Together they mean:

• that he had told

This is just how indirect statement works in Latin: the subject becomes accusative, and the verb becomes infinitive.