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

Matertera means a maternal aunt, specifically your mother’s sister.

Latin can be more precise than English here:

• matertera = mother’s sister
• amita = father’s sister

So Latin is not just saying aunt in a general sense; it is identifying exactly which side of the family she is from.

Postridie means the next day or on the following day.

It is not a noun here, so you do not need to assign it a case in the way you would for a normal noun in the sentence. It functions as an adverb of time.

So:

• Postridie = the next day

It sets the time for the main action: the aunt promises the next day.

Promittit is the present active indicative, third person singular: she promises.

Latin often uses the present simply because the sentence is being presented as a straightforward statement. Depending on context, English might translate it in different ways:

• she promises
• the aunt promises
• in narrative, sometimes even she promised, if the author is using a vivid present

But grammatically, the form itself is present: promittit = she promises.

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

That construction is:

• accusative subject
  • infinitive

Here:

• se = herself / she in the accusative
• daturam esse = to be going to give / would give

So promittit se ... daturam esse literally means:

• she promises herself to be going to give ...

More naturally in English:

• she promises that she will give ...

This is very common Latin grammar.

Se is the reflexive pronoun, used when the subject of the main verb and the subject of the indirect statement are the same person.

Here the main subject is matertera:

• matertera promittit = the aunt promises

And the one who will give the gift is also the aunt. So Latin uses se:

• she promises that she herself will give

If Latin used eam, that would normally mean that some other woman/girl would give it, not the aunt herself.

Because daturam agrees with se, and se refers to matertera, which is feminine.

The form comes from the future active participle of do, dare:

• masculine: daturus
• feminine: datura
• neuter: daturum

Since the aunt is female:

• se daturam esse = that she will give

So the feminine ending is required.

It is the future infinitive of do in indirect statement.

Latin does not usually say that she will give by using a finite verb after promittit. Instead, it uses:

• se = accusative subject
• daturam esse = future infinitive

This specifically shows action future relative to the main verb:

• promittit se daturam esse = she promises that she will give

So daturam esse is how Latin expresses the future action inside the indirect statement.

Because nepti is the indirect object of dare.

The verb do, dare often takes:

• a direct object = the thing given
• an indirect object in the dative = the person receiving it

Here:

• parvum donum = the small gift (direct object)
• nepti = to the granddaughter / niece (indirect object)

So:

• se nepti parvum donum daturam esse
• that she will give a small gift to her granddaughter / niece

Nepti is the dative singular of neptis.

Neptis can mean:

• granddaughter
• sometimes niece, depending on context

So nepti means:

• to the granddaughter
• or to the niece

The exact English choice depends on the meaning already supplied in context.

Because it is the direct object of dare.

The thing being given is the gift:

• donum = gift
• parvum = small, agreeing with donum

Both are:

• neuter singular
• accusative

So parvum donum means a small gift and is the thing the aunt promises to give.

Dummodo means provided that, so long as, or as long as only.

It introduces a condition:

• dummodo illa matri auxilium ferat
• provided that she gives help to her mother
• more naturally: provided that she helps her mother

So the promise is conditional.

Because dummodo commonly introduces a clause with the subjunctive.

Here ferat is:

• present subjunctive
• third person singular
• from fero, ferre

So:

• dummodo ... ferat = provided that ... she may bring / she helps

In smoother English, we usually do not show the subjunctive directly; we simply translate the whole clause as:

• provided that she helps her mother

Yes. Ferat comes from the irregular verb fero, ferre, tuli, latum, meaning to bear, carry, bring.

Because fero is irregular, some of its forms may look unfamiliar. Here:

• ferat = present subjunctive, third singular
• literally: she may carry / bring

In the expression auxilium ferre, the meaning is idiomatic:

• auxilium ferre alicui = to bring help to someone
• more naturally: to help someone

So matri auxilium ferat means she helps her mother.

Because auxilium ferre is a standard Latin idiom.

Literally it means:

• to bring help

But it is commonly used in the sense:

• to help
• to give assistance

So:

• matri auxilium ferat = she gives help to her mother
• natural English: she helps her mother

Learners often need to recognize this as a set phrase rather than translating each word too mechanically.

Because matri is the person receiving the help, and with auxilium ferre that person is put in the dative.

So:

• matri = to her mother
• auxilium ferre matri = to bring help to her mother

In smoother English, we say:

• to help her mother

In context, illa most naturally refers to the girl already mentioned, probably the niece/granddaughter.

So the sense is:

• the aunt promises to give the girl a small gift, provided that that girl helps her mother

Latin uses illa here as a demonstrative pronoun, literally that girl / she. It can help make the subject of the subordinate clause clearer or more emphatic than a simple implied subject would.

Latin often leaves subjects unstated, because the verb ending already tells you the person and number. So in theory the clause could have been just:

• dummodo matri auxilium ferat

However, adding illa can do a few useful things:

• make the subject clearer
• add a little emphasis
• distinguish that person from another female already in the sentence

Since both matertera and nepti are feminine, illa helps point to the intended girl/woman in the conditional clause.

Grammatically, a pronoun can sometimes be ambiguous, so context matters. But here the most natural reading is that illa refers to the niece/granddaughter, not the aunt.

Why?

Because the condition attached to the gift usually concerns the recipient:

• the aunt will give the gift
• provided that the girl helps her mother

Also, matri likely means her own mother, which fits the niece/granddaughter better in the context.

So while Latin pronouns can sometimes be less explicit than English would like, the probable reference here is the younger female, not the aunt.

Because Latin does not normally use a separate word like English that in this kind of indirect statement.

English says:

• she promises that she will give ...

Latin instead uses the accusative-and-infinitive construction:

• promittit se ... daturam esse

So the idea of English that is built into the construction itself, rather than expressed by a separate word.

A helpful way to break it up is:

• Postridie = time expression: the next day
• matertera promittit = main clause: the aunt promises
• se nepti parvum donum daturam esse = indirect statement: that she will give the girl a small gift
• dummodo illa matri auxilium ferat = conditional clause: provided that she helps her mother

So the sentence is built from:

1. a time word
2. a main verb
3. an indirect statement
4. a condition introduced by dummodo

That is a very typical Latin pattern.