La filino promesis, ke ŝi venos kun sia filo al la naskiĝtago de la avino.

In Esperanto, a comma is normally used before a subordinate clause introduced by ke (that).

So:

• La filino promesis, ke ŝi venos...
• The daughter promised that she would come...

This comma is standard and very common in Esperanto writing.

Ke means that and introduces a content clause: it tells you what was promised.

• La filino promesis = The daughter promised
• ke ŝi venos... = that she will come / that she would come...

So the whole clause after ke is the thing she promised.

Venos is the ordinary future tense of veni (to come).

• venos = will come
• venus = would come, in a more conditional or less certain sense

In Esperanto, after a past verb like promesis, it is very normal to keep the future form if the action is still future relative to that past moment:

• Ŝi promesis, ke ŝi venos. = She promised that she would come.

English often “backshifts” tenses, but Esperanto usually keeps the tense that matches the actual time relationship more directly.

Yes, ŝi is needed here.

Esperanto verbs do not change according to person, so venos by itself only means will come. It does not tell you who will come. Because of that, the subject pronoun usually has to be stated:

• ke ŝi venos = that she will come

Unlike languages such as Spanish or Italian, Esperanto is generally not a “drop-the-subject” language.

This is one of the most important grammar points in Esperanto.

Sia is the reflexive possessive. It refers back to the subject of the clause it is in.

In the clause:

• ke ŝi venos kun sia filo

the subject is ŝi. So sia filo means her own son — the son of the woman who will come.

If you said ŝia filo, that would usually mean some other female person’s son, not the subject’s own son.

So here:

• ŝi ... kun sia filo = she ... with her own son

Grammatically, sia refers to the subject of its own clause, which here is ŝi.

That said, in this sentence ŝi clearly refers back to la filino, so in meaning the son is the daughter’s son.

The important rule is:

• Look at the subject of the clause where sia appears.
• Here that subject is ŝi.

So sia filo is correct because it belongs to ŝi, the subject of venos.

Kun means with.

So:

• kun sia filo = with her son

It is a prepositional phrase showing accompaniment: who is coming together with her.

Also note that after a preposition like kun, the noun does not take -n unless there is some special reason involving direction or measure. So just:

• kun sia filo not
• kun sian filon

Al usually means to and marks the destination or target.

Here it means she is coming to the birthday celebration:

• al la naskiĝtago = to the birthday / to the birthday party

Esperanto often uses naskiĝtago not only for the date itself but also for the birthday occasion or celebration, depending on context.

Because al already marks the role of the noun.

In Esperanto, nouns often take -n when they are direct objects. But after a preposition like al, you normally do not add -n:

• al la naskiĝtago = to the birthday

So naskiĝtago is not the direct object here. It is the object of the preposition al.

De means of or from, and here it shows possession or association:

• la naskiĝtago de la avino = the grandmother’s birthday

So literally it is the birthday of the grandmother.

This is a very common Esperanto way to express possession, especially when the possessor is a full noun phrase.

Usually, de la avino is the clearest and most natural way here.

You might sometimes see a related adjectival form in Esperanto, but for ordinary possession with a noun, de is standard and easy:

• la libro de la knabo = the boy’s book
• la naskiĝtago de la avino = the grandmother’s birthday

So for learners, de is the safest and most natural choice.

Because Esperanto uses the suffix -in- to mark female beings.

• filo = son

• filino = daughter

• av(o) is the root behind avino
• avino = grandmother

So:

• fil-in-o = female child = daughter
• av-in-o = female grandparent = grandmother

This is a very common word-building pattern in Esperanto.

It is a compound word:

• naskiĝi = to be born
• tago = day

So:

• naskiĝtago = birth-day = birthday

Esperanto very often builds words this way by combining meaningful parts into one compound.

La is the definite article, meaning the.

So:

• La filino = the daughter

Using la shows that the speaker has a specific daughter in mind, not just any daughter. Since the English meaning is probably something like The daughter promised..., la fits naturally.

Yes, Esperanto word order is fairly flexible, but the original order is the most neutral and natural.

For example, you could move some parts for emphasis, but:

• La filino promesis, ke ŝi venos kun sia filo al la naskiĝtago de la avino.

is clear and standard.

Because Esperanto marks grammatical relationships with endings and prepositions, word order can vary more than in English, but beginners should usually stick to the most straightforward order.

It means both ideas together:

• ŝi venos = she will come
• kun sia filo = with her son
• al la naskiĝtago de la avino = to the grandmother’s birthday

So the meaning is that the daughter promised she would come to the grandmother’s birthday, and her son would come with her.

Not much in normal reading. In this sentence, sia filo very naturally means the son of the woman who will come.

Because sia points to the subject of the clause, Esperanto is actually helping prevent ambiguity here. If the writer had used ŝia filo, readers might wonder whether it meant the daughter’s son or another woman’s son.

So sia is the precise and correct choice.