Puer matri de somnio tristi narrat.

Because puer is in the nominative case, which is the case usually used for the subject of the sentence.

Here:

• puer = the boy
• narrat = tells / is telling

So puer narrat means the boy tells.

Its dictionary form is puer, pueri.

Because narrat commonly takes the person being told in the dative case.

So:

• matri = to the mother
• not matrem, which would be accusative

In this sentence, the structure is:

• puer = subject
• matri = indirect object, to the mother
• de somnio tristi = about a sad dream
• narrat = tells

A very natural way to understand it is: The boy tells the mother about a sad dream.

Because de is a preposition that often means about or concerning, and it takes the ablative case.

So:

• de = about
• somnio = ablative singular of somnium
• tristi = ablative singular of tristis, agreeing with somnio

Together:

• de somnio tristi = about a sad dream

Because adjectives in Latin must agree with the noun they describe in:

• gender
• number
• case

Here:

• somnio is ablative singular
• so the adjective must also be ablative singular
• therefore: tristi

Even though somnium is neuter, the ablative singular of tristis, triste is tristi for masculine, feminine, and neuter.

So:

• somnium triste = a sad dream if it were nominative or accusative
• de somnio tristi = about a sad dream because de requires the ablative

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

So matri literally means:

• to the mother

But in many contexts, English naturally says:

• to his mother

Latin does have words like suus, sua, suum for his/her/its own, but they are not always necessary.

So the sentence could be understood as:

• The boy tells his mother about a sad dream even though Latin simply says to mother.

Because Latin has no articles.

English distinguishes between:

• the boy
• a boy

Latin usually just says puer, and the context tells you whether to translate it as:

• the boy
• a boy
• sometimes just boy in a very literal gloss

The same is true for:

• matri = to the mother / to a mother
• somnio tristi = a sad dream / the sad dream

Not as much as in English.

Latin relies heavily on case endings, so the roles of the words are shown by their forms, not mainly by their position.

So Puer matri de somnio tristi narrat can be rearranged in several ways without changing the basic meaning, for example:

• Matri puer de somnio tristi narrat
• De somnio tristi puer matri narrat
• Puer de somnio tristi matri narrat

The emphasis may shift slightly, but the core meaning stays the same because:

• puer is nominative
• matri is dative
• somnio tristi is ablative after de

Narrat is:

• 3rd person singular
• present tense
• active voice
• from the verb narro, narrare

So it means:

• he tells
• she tells
• it tells

Here, because the subject is puer, it means:

• the boy tells

Here are the usual dictionary forms:

• puer, pueri = boy
• mater, matris = mother
• somnium, somnii = dream
• tristis, triste = sad
• narro, narrare, narravi, narratus = tell, relate

These dictionary forms help you recognize which declension or conjugation each word belongs to.

Yes, often it could.

A sentence like:

• Puer somnium triste matri narrat

would mean:

• The boy tells his mother a sad dream or more naturally,
• The boy tells his mother about a sad dream

With de + ablative, Latin makes the about idea more explicit:

• de somnio tristi = about a sad dream

So both patterns are possible, but this sentence specifically uses de.

Because they are in different cases and belong to different declensions.

• puer is nominative singular, the subject
• matri is dative singular, the indirect object

Also, their dictionary forms come from different noun patterns:

• puer, pueri is a 2nd declension noun
• mater, matris is a 3rd declension noun

So their endings change differently:

• puer stays puer in the nominative singular
• mater becomes matri in the dative singular

Not by itself.

Grammatically, matri just means:

• to mother
• to the mother
• to a mother

Whether it is his mother, the boy’s mother, or someone else’s mother depends on the context.

English often adds his because it sounds natural, but Latin does not have to say it.

So the Latin is slightly less specific than the smooth English translation.