Mater promittit se post paucos menses aliud convivium celebraturam esse, si pax manserit.

Because Latin usually does not use a separate that-clause after verbs like promittit. Instead, it uses an indirect statement construction:

• se ... celebraturam esse

This is the Latin way of saying that she will celebrate ...

So the structure is:

• Mater promittit = Mother promises
• se ... celebraturam esse = that she will celebrate ...

English uses that; Latin often uses accusative + infinitive instead.

Se is a reflexive pronoun, and here it refers back to the subject of the main verb, mater.

So:

• Mater promittit se ... celebraturam esse
• literally: Mother promises herself ... about to celebrate
• naturally: Mother promises that she will celebrate ...

Latin uses se instead of a regular third-person pronoun when the person in the indirect statement is the same as the subject of the main verb.

So se = mother herself, not some other woman.

Because in a Latin indirect statement, the subject of the infinitive is put in the accusative.

Here the infinitive is:

• celebraturam esse = to be going to celebrate / will celebrate

And the subject of that infinitive is:

• se

So even though se means she, it appears in the accusative because Latin indirect statement works like this:

• [accusative subject] + [infinitive]

That is why Latin has se celebraturam esse, not a nominative form.

Because the verb is inside an indirect statement after promittit.

If this were a direct statement, Latin could say:

• Mater aliud convivium celebrabit.
• Mother will celebrate another banquet.

But after promittit, Latin changes the future verb into a future infinitive:

• celebraturam esse

So:

• promittit se celebraturam esse
• she promises that she will celebrate

This is normal Latin grammar after verbs of saying, thinking, knowing, promising, and similar verbs.

It is the future active infinitive of celebro, celebrare.

It is formed from:

• the future participle: celebratura
• plus esse

So:

• celebratura esse = to be going to celebrate / to celebrate in the future

In this sentence it appears as celebraturam esse because it agrees with se, which is feminine singular accusative.

So the agreement is:

• se = feminine singular accusative
• celebraturam = feminine singular accusative

This shows that mother is the one who will do the celebrating.

Because the future participle must agree with the person it describes.

Here it describes se, which refers to mater:

• mater = feminine
• therefore the participle must be feminine too

So Latin uses:

• celebraturam esse = feminine accusative singular

If the subject were masculine, you would expect:

• celebraturum esse

This agreement is one of the clues that se refers back to mater.

Because post takes the accusative case.

So:

• post = after
• paucos menses = accusative plural

More specifically:

• paucos agrees with menses
• menses is accusative plural of mensis

Together, post paucos menses means after a few months.

A learner may be tempted to think of English in a few months, but Latin post specifically means after.

Because aliud is an adjective, and it must agree with convivium in:

• gender
• number
• case

Here:

• convivium is neuter singular accusative
• so the adjective must also be neuter singular accusative
• therefore: aliud convivium

So aliud means another or a different, and it matches convivium grammatically.

Here manserit is most naturally understood as a future perfect indicative.

It comes from maneo, manere = remain.

So:

• si pax manserit = if peace has remained / if peace still holds / if peace lasts

Latin often uses the future perfect in a si-clause when talking about something that must be true or completed before another future action happens.

So the sense is:

• Mother promises that she will celebrate another banquet, if peace still holds.

In English, we often translate this more simply with a present tense:

• if peace remains
• if peace lasts

even though the Latin form is future perfect.

Because this is a straightforward future condition, not a hypothetical or contrary-to-fact one.

Latin commonly uses:

• si
  • indicative

for ordinary conditions. Here the speaker is talking about a real future possibility:

• if peace remains

So the indicative is exactly what we would expect.

The subjunctive would suggest a different kind of clause or a different nuance, but this sentence is simply setting a condition on the promised future event.

The si clause belongs with the content of the promise. In other words, what the mother promises is:

• se post paucos menses aliud convivium celebraturam esse, si pax manserit

So she is not making two separate statements. She is promising one future action, and that action is conditional:

• that she will celebrate another banquet after a few months, if peace remains

This is useful to notice because the sentence contains both:

• an indirect statement: se ... celebraturam esse
• a conditional clause: si pax manserit

Latin can combine those structures smoothly in one sentence.

A helpful way to break it apart is this:

• Mater promittit = main clause
• se ... celebraturam esse = indirect statement
• post paucos menses = time phrase
• aliud convivium = object of celebraturam esse
• si pax manserit = condition

So the grammar works like this:

1. Mater is the subject.
2. promittit is the main verb.
3. se is the accusative subject of the infinitive.
4. celebraturam esse expresses the future action.
5. aliud convivium is what she will celebrate.
6. post paucos menses tells when.
7. si pax manserit gives the condition.

Seeing that structure makes the sentence much easier to read.