Postridie matertera promittit se nepti parvum donum daturam esse, dummodo illa matri auxilium ferat.

Because Latin regularly uses an indirect statement after verbs of saying, thinking, knowing, and promising.

After promittit (she promises), Latin does not usually say that she will give with a finite verb. Instead, it uses:

• an accusative subject: se
• plus an infinitive: daturam esse

So promittit se ... daturam esse literally works like she promises herself to be going to give..., which is the normal Latin way to say she promises that she will give...

Se is the reflexive pronoun, meaning herself in this sentence.

It refers back to the subject of the main verb, which is matertera. So:

• matertera promittit = the aunt promises
• se ... daturam esse = that she herself will give

Latin uses se when the subject of the infinitive clause is the same as the subject of the main verb.

There are two things going on here.

1. It is future infinitive
   • dare = to give
   • daturam esse = to be going to give / that she will give

Since the promise is about a future action, Latin uses the future active infinitive.

2. It agrees with the implied subject
   • The subject is se, referring to matertera
   • Since matertera is feminine singular, the participle must also be feminine singular

So:

• masculine: daturum esse
• feminine: daturam esse

That is why daturam esse is correct.

Because they belong to different clauses.

• matertera is the subject of the main verb promittit, so it is nominative.
• se is the subject of the infinitive daturam esse in an indirect statement, and in Latin the subject of an indirect statement goes into the accusative.

This is a standard feature of Latin syntax: accusative + infinitive.

Because nepti is the indirect object of daturam esse.

With dare (to give), Latin commonly uses:

• accusative for the thing given
• dative for the person receiving it

So here:

• parvum donum = the thing given
• nepti = the person receiving it

That is why nepti is dative.

It is accusative singular.

That is because it is the direct object of daturam esse:

• donum is a neuter second-declension noun
• parvum agrees with it in gender, number, and case

So parvum donum means a small gift, and it is accusative because it is the thing being given.

Postridie is an adverb meaning on the next day / the next day.

It does not need a preposition. Latin often uses single adverbs for time expressions where English may use a phrase.

So postridie simply sets the time for the main action: the promise happens the next day.

Because dummodo means provided that / as long as, and it often introduces a clause with the subjunctive.

So:

• dummodo = provided that
• ferat = present subjunctive of ferre

This is a very common construction in Latin for conditions with a sense like on condition that.

Ferat is present subjunctive active, third person singular, from ferre.

It is subjunctive because it comes after dummodo.

Its tense is present, but in a clause like this the important point is usually not literal present time in English. The clause expresses the condition attached to the promise: provided that she gives/brings help to her mother.

So the subjunctive here is more about the kind of clause than about a dramatic difference in time.

Latin often leaves pronouns out when the verb already shows the subject. So ferat by itself could mean she may bring / she brings.

Using illa adds some emphasis or clarity. It can mean something like:

• she
• that girl/woman
• that one

Here it helps mark the subject of the conditional clause clearly, especially since there is already another female person in the sentence (matertera). So illa helps the reader keep the two women distinct.

Grammatically, illa refers to the female person who is expected to help matri. In context, that will be the person previously mentioned as nepti.

So the structure is:

• the aunt promises that she will give a small gift to the niece/granddaughter
• provided that that girl/she helps her mother

Latin sometimes uses a demonstrative like illa to point back to someone already mentioned and to keep the sentence clear.

Because auxilium ferre alicui means to bring/give help to someone.

So in this expression:

• auxilium is the direct object
• matri is the person receiving the help, so it is dative

This is similar to the way dare works with a direct object plus a dative recipient.

Yes. Auxilium ferre is a common Latin idiom meaning to bring help, that is, to help or to give assistance.

Literally:

• auxilium = help
• ferre = to carry, bring

But together they function as a natural expression for giving assistance.

So matri auxilium ferat means that she should give help to her mother or more naturally that she help her mother.

Latin word order is much more flexible than English word order because the endings show the grammatical roles.

This sentence can place words for emphasis, balance, or style:

• Postridie comes first to set the scene in time
• matertera appears early as the main subject
• se ... daturam esse stays together as the indirect statement
• dummodo introduces the condition at the end

English depends much more on position, but Latin depends more on case endings and verb forms. So even if the order feels unusual, the grammar still makes each role clear.

Because the main verb and the future action are different things.

• promittit describes the act of promising, and that act is presented as present: she promises
• daturam esse describes the later action: that she will give

So Latin uses:

• present tense for the promise itself
• future infinitive for the thing promised

This is exactly how Latin distinguishes the present act of promising from the future act of giving.