Se neniu el niaj geamikoj estus povinta veni, niaj geavoj estus restintaj hejme.

Because this is a counterfactual or unreal condition.

In English, we usually say something like If none of our friends had been able to come, our grandparents would have stayed home. English uses a past perfect form in the if-clause and would have in the main clause.

Esperanto often handles this by using the conditional in both clauses:

• Se ... estus ...
• ..., ... estus ...

The idea is that both parts belong to an unreal situation. The past sense is then shown by the participles such as povinta and restintaj.

So this pattern is normal:

• Se X estus okazinta, Y estus okazinta.

It is a compound form made of:

• estus = conditional of esti
• povinta = past active participle of povi
• veni = infinitive

Literally, it is something like would have been able to come.

So:

• povi = to be able, can
• povinta = having been able
• estus povinta = would have been able

Then adding veni gives:

• estus povinta veni = would have been able to come / could have come

This construction clearly points to an unreal possibility in the past.

Because povus veni usually means could come or would be able to come, and that often sounds more like a present/future or a less clearly time-marked hypothetical.

By contrast:

• povus veni = could come / would be able to come
• estus povinta veni = could have come / would have been able to come

So in this sentence, estus povinta veni is better because the whole sentence is about an unreal past situation.

Restintaj is the past active participle of resti.

• resti = to stay, remain
• restinta = having stayed, having remained
• restintaj = same idea, but plural

In Esperanto, participles used with esti behave like adjectives, so they agree with the subject in number and, when relevant, case.

Here the subject is niaj geavoj, which is plural, so the participle must also be plural:

• geavoj → plural
• therefore restintaj → plural

So:

• estus restintaj hejme = would have stayed at home

Because restus hejme usually means would stay home or would remain at home, without clearly expressing would have stayed in the past.

The sentence wants a past counterfactual result, so Esperanto uses:

• estus restintaj = would have stayed

Compare:

• Ili restus hejme = They would stay home.
• Ili estus restintaj hejme = They would have stayed home.

So the -int- participle is what gives the clear past/completed sense.

-int- is the marker for the past active participle.

Very roughly:

• -ant- = doing
• -int- = having done
• -ont- = about to do / going to do

So:

• povinta = having been able
• restinta = having stayed

In this sentence, -int- helps show that the ability to come and the staying at home are viewed as already completed within that unreal past scenario.

Because neniu is grammatically singular.

Even though it refers to zero members of a group, Esperanto treats neniu as a singular word, like no one in English.

So you get:

• neniu ... estus povinta
  not
• neniu ... estus povintaj

The head of the phrase is neniu, and el niaj geamikoj just tells you which group is being talked about.

Here el means out of or from among. It creates a partitive idea.

So:

• neniu el niaj geamikoj = none of our friends
• literally: no one out of our friends

This is a very common Esperanto pattern:

• unu el ili = one of them
• kelkaj el la studentoj = some of the students
• neniu el ni = none of us

Ge- indicates a mixed-sex group, especially when both males and females are included.

So:

• geavoj = grandparents
• geamikoj = friends of both sexes / a mixed-gender group of friends

With family words like gepatroj, geavoj, gefratoj, this is extremely standard.

With non-family words like amikoj, modern Esperanto often uses ge- when the speaker wants to make the mixed group explicit. If gender does not matter, many speakers would simply say amikoj.

Because each noun phrase normally needs its own modifier.

The sentence has two separate noun phrases:

• niaj geamikoj
• niaj geavoj

Repeating niaj makes the sentence clear and natural. If you removed the second one, geavoj would just mean grandparents, without explicitly saying whose grandparents they are.

So the repetition is normal and expected.

Because hejme means at home, while hejmen means to home / homeward.

Here the idea is location, not movement:

• resti hejme = stay at home
• iri hejmen = go home

Since the grandparents would have remained in that place, hejme is the correct form.

Yes. Esperanto word order is fairly flexible.

For example, this would also be correct:

• Niaj geavoj estus restintaj hejme, se neniu el niaj geamikoj estus povinta veni.

The original order puts the condition first, which is very natural. The alternative puts the result first, which is also normal.

In both versions, the comma between the two clauses is standard.