Puer matri narrat se in somnio montem altissimum vidisse.

Because narrat often takes the person being told in the dative: alicui narrare = to tell someone.

So in this sentence:

• puer = the boy (subject, nominative)
• matri = to his mother / to the mother (dative)
• narrat = tells

A native English speaker may expect mother to be a direct object, but Latin treats it as an indirect object here: The boy tells his mother ...

This is the very common Latin construction called the indirect statement or accusative-and-infinitive construction.

After verbs of saying, thinking, knowing, perceiving, and similar verbs, Latin often uses:

• an accusative subject
• plus an infinitive verb

So:

• se = the subject of the reported statement, in the accusative
• vidisse = to have seen

Literally, Latin says something like:

The boy tells his mother himself to have seen a very high mountain in a dream.

But natural English turns that into:

The boy tells his mother that he saw a very high mountain in a dream.

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

So:

• puer narrat se vidisse = the boy says that he himself saw
• puer narrat eum vidisse = the boy says that he saw, where him means some other male, not the boy

Since the one doing the telling is also the one who saw the mountain, Latin uses se.

In an indirect statement, the subject of the reported clause goes into the accusative, not the nominative.

So although se is logically the subject of vidisse, Latin grammar requires it to be accusative because it is the subject of an infinitive in indirect statement.

Compare:

• direct statement: ego vidi = I saw
• indirect statement: dicit se vidisse = he says that he saw

English keeps he as a subject form, but Latin changes it to the accusative in this construction.

Vidisse is the perfect active infinitive of videre. It means to have seen.

Latin infinitive tense in indirect statement shows time relative to the main verb, not absolute time by itself.

Here:

• narrat = he tells / is telling (present)
• vidisse = action earlier than the telling

So the sense is:

He tells his mother that he had seen / saw a very high mountain in a dream.

If Latin used videre, that would mean the seeing is happening at the same time as the telling:

• narrat se videre = he says that he sees / is seeing

Because it is the direct object of vidisse.

Inside the indirect statement:

• se = subject of vidisse
• montem altissimum = object of vidisse

So the inner clause is basically:

se montem altissimum vidisse = that he saw a very high/highest mountain

Even though vidisse is an infinitive, it can still take a normal direct object in the accusative.

Grammatically, altissimum is the superlative of altus, so its basic form means highest or tallest.

But in Latin, superlatives are often used in a more general intensifying sense, especially in ordinary prose. So mons altissimus can mean either:

• the highest mountain
• a very high mountain

Without more context, either is possible. Many learners first think superlatives must always be strictly literal, but Latin often uses them more flexibly than English does.

Because in with the ablative usually expresses location or state within something, while in with the accusative often expresses motion into something.

Here in somnio means:

• in a dream
• while dreaming
• within the dream experience

So the ablative makes sense because there is no movement into a dream being emphasized.

Compare the general pattern:

• in urbe = in the city
• in urbem = into the city

Latin often leaves out possessive words like his, her, or their when the meaning is obvious from context.

So matri can naturally be understood as to his mother here, because the subject is puer, and it is very natural to assume the mother is his unless context suggests otherwise.

Latin does have possessives:

• eius matri = to his/her mother
• suae matri = to his own mother

But they are not always necessary.

Latin word order is freer than English word order because case endings show the grammatical roles.

So:

• Puer matri narrat se in somnio montem altissimum vidisse
• Puer se montem altissimum in somnio vidisse matri narrat
• Matri puer narrat se in somnio montem altissimum vidisse

can all express essentially the same basic meaning.

That said, word order still affects emphasis and style. In the original sentence:

• Puer comes first, setting up the subject
• matri narrat quickly gives the main action
• se in somnio montem altissimum vidisse then gives the content of what he tells

So the order is quite natural and readable.

A useful way to break it up is:

• Puer = subject of the main clause
• matri = indirect object of the main clause
• narrat = main verb
• se in somnio montem altissimum vidisse = the content being told

Then inside that reported content:

• se = subject of the infinitive
• in somnio = adverbial phrase
• montem altissimum = direct object
• vidisse = infinitive verb

So the sentence has two layers:

Main clause: The boy tells his mother ...

Indirect statement: ... that he saw a very high mountain in a dream.

That two-layer structure is one of the most important things for learners to get comfortable with in Latin.