Puella cistam aperit et annulum parvum inter epistulas invenit.

Because puella is in the nominative singular, the case normally used for the subject of a sentence.

Also, both verbs, aperit and invenit, are third-person singular, so they match a singular subject: the girl / she.

So puella is the one doing both actions.

Because cistam is the direct object of aperit: it is the thing being opened.

In Latin, a direct object usually goes in the accusative case.
Cista is nominative singular, while cistam is accusative singular.

So:

• cista = box as a subject
• cistam = box as an object

Because annulum parvum means small ring, and the whole phrase is the direct object of invenit.

That means annulum must be accusative singular. The adjective parvum has to agree with annulum in:

• gender: masculine
• number: singular
• case: accusative

So:

• annulus parvus = a small ring as subject
• annulum parvum = a small ring as object

In Latin, adjectives can come before or after the noun much more freely than in English.

So annulum parvum and parvum annulum are both possible.
Here, annulum parvum is a perfectly normal word order.

Latin word order often depends on style, emphasis, and rhythm, not on a strict rule like English.

Because the preposition inter takes the accusative case.

So after inter, Latin expects an accusative form:

• epistulae = nominative plural
• epistulas = accusative plural

That is why the sentence has inter epistulas.

It can mean either, depending on context.

With a plural noun like epistulas (letters), English often uses among more naturally:

• among the letters

But between the letters is not impossible in some contexts.

The important grammar point is that inter still takes the accusative.

Because both are third-person singular present active indicative forms.

They mean:

• aperit = she/he/it opens
• invenit = she/he/it finds

Since puella is the subject, we understand them here as she opens and she finds.

The ending -it tells you the verb is:

• present tense
• third person
• singular

Because Latin has no articles.

So puella can mean:

• girl
• a girl
• the girl

And cistam can mean:

• a box
• the box

The exact sense comes from context, not from a separate word like a or the.

It goes with both verbs.

The sentence structure is:

• Puella cistam aperit
• et annulum parvum inter epistulas invenit

The subject puella is understood for both actions: the girl opens the box and finds the small ring among the letters.

Latin does not need to repeat the subject if it stays the same.

No. Latin word order is much freer than English word order because the case endings show how the words function.

So this sentence could be rearranged in various ways without changing the basic meaning, for example:

• Puella cistam aperit et annulum parvum inter epistulas invenit
• Puella aperit cistam et invenit annulum parvum inter epistulas
• Annulum parvum puella inter epistulas invenit

The endings still show:

• puella = subject
• cistam, annulum = direct objects
• epistulas = object of inter

Word order in Latin often affects emphasis, not basic grammar.