Se la vetero estus pli bela, niaj geavoj sidus en la ĝardeno antaŭ la domo.

Because the sentence expresses an unreal or hypothetical situation.

In Esperanto, -us is the conditional mood. It is used for things like would, would be, would do, and often also for if ... were/would ... in unreal situations.

So here:

• Se la vetero estus pli bela = if the weather were nicer
• niaj geavoj sidus = our grandparents would sit / would be sitting

A very important difference from English is that Esperanto often uses -us in both parts of this kind of unreal conditional sentence.

Estus is the conditional form of esti (to be).

• estas = is / are
• estis = was / were
• estos = will be
• estus = would be / were

In this sentence, estus means something like would be or, in natural English, were:

• Se la vetero estus pli bela = if the weather were nicer

So it marks the weather as an imagined condition, not a real current fact.

It can mean either, depending on context.

Sidi means to sit or to be sitting. Esperanto often does not force the same distinction English does between:

• would sit
• would be sitting

So:

• sidus can mean would sit
• but in this sentence it very naturally feels like would be sitting

That is normal in Esperanto. The simple verb often covers both ideas.

Both are possible in some contexts, but they mean different things.

• bela = beautiful, lovely, pleasant-looking
• bona = good

So:

• pli bela vetero suggests nicer / more beautiful / more pleasant weather
• pli bona vetero suggests better weather more generally

In real usage, weather is often described with words like bela because it suggests pleasant, attractive weather—good for being outside.

Ge- is a prefix used for a mixed-sex group of relatives or people.

• avo = grandfather
• avino = grandmother
• geavoj = grandparents

So niaj geavoj means our grandparents.

A key point: ge- is normally used with a plural ending, because it refers to a group.

Because geavoj is plural, so the possessive adjective must also agree in number.

• nia = our (singular noun)
• niaj = our (plural noun)

Since geavoj ends in -oj and is plural, we need:

• niaj geavoj = our grandparents

This is normal Esperanto adjective agreement: adjectives and similar words match the noun in number and case.

Because these words are showing location, not direction toward something.

• en la ĝardeno = in the garden
• antaŭ la domo = in front of the house

The accusative -n is often used for a direct object, and also for motion toward a place.

Compare:

• Ili sidas en la ĝardeno. = They are sitting in the garden.
  → location, so no -n
• Ili iras en la ĝardenon. = They are going into the garden.
  → movement into it, so ĝardenon

Same with antaŭ la domo: it describes where the garden is, not movement.

Yes. Antaŭ can mean both:

• before in time
• in front of in space

In this sentence, the context is clearly spatial:

• la ĝardeno antaŭ la domo = the garden in front of the house

Esperanto often lets context tell you which meaning is intended.

Grammatically, it most naturally describes the garden:

• en la ĝardeno antaŭ la domo = in the garden that is in front of the house

In practice, the whole phrase tells you where the grandparents would be sitting. But inside the phrase, antaŭ la domo most directly modifies ĝardeno.

So the idea is not just any garden, but the garden in front of the house.

Esperanto uses la as its definite article, like English the.

In this sentence:

• la vetero = the weather
  Weather is often treated as a specific understood thing in context.
• la ĝardeno = the garden
  A specific garden is meant.
• la domo = the house
  Again, a specific house is meant.

Esperanto has only one article, la, and it does not change for gender or number.

Yes, but it would mean something different.

• Se la vetero estas pli bela, niaj geavoj sidas en la ĝardeno...
  = If the weather is nicer, our grandparents are sitting in the garden...
  This sounds like a real or possible situation.

• Se la vetero estus pli bela, niaj geavoj sidus...
  = If the weather were nicer, our grandparents would be sitting...
  This is hypothetical or contrary to fact.

So estus ... sidus is the right choice when the speaker is imagining a situation that is not currently true.

Yes, but it usually does not need to.

Esperanto can be more explicit with forms like:

• sidantaj = sitting
• estus sidantaj = would be sitting

So you could say something like:

• Se la vetero estus pli bela, niaj geavoj estus sidantaj en la ĝardeno...

But that is heavier and usually unnecessary. Simple sidus already communicates the idea well.

Esperanto often prefers the simpler form unless there is a special reason to emphasize the ongoing action.

The word order is very natural and straightforward:

• Se la vetero estus pli bela = if the weather were nicer
• niaj geavoj sidus = our grandparents would sit / would be sitting
• en la ĝardeno antaŭ la domo = in the garden in front of the house

Esperanto word order is fairly flexible, but this sentence uses a common, neutral order:

1. condition
2. main clause
3. place information

Because Esperanto marks grammar clearly with endings, the sentence could be rearranged somewhat, but this version is the most ordinary and easiest for a learner.