Mia amikino estus gvidinta nin preter la malnova kvartalo, se ni estus alvenintaj pli frue.

It means would have guided or would have led.

This is a past counterfactual form: it talks about something that did not actually happen, but would have happened under different circumstances.

So here:

• Mia amikino estus gvidinta nin...
  = My female friend would have guided us...

Because estus gvidinta is more specific.

• gvidus = would guide / would lead
• estus gvidinta = would have guided / would have led

The form with estus + participle shows that the action would have been completed in the past.

So in this sentence, the speaker is not talking about a general or present/future possibility. They are talking about an unreal past situation:

• we did not arrive earlier
• therefore, she did not guide us past the old quarter

Because Esperanto often uses this kind of form in both parts of an unreal past condition.

• se ni estus alvenintaj pli frue
  = if we had arrived earlier

English uses different-looking forms:

• if we had arrived earlier
• she would have guided us

Esperanto often expresses both ideas with estus + -int- participle:

• se ni estus alvenintaj...
• ŝi estus gvidinta...

So even though estus alvenintaj may look literally like would have arrived, in a se clause it is understood as had arrived.

-int- is the active past participle.

It shows that the subject is someone who has done the action.

Examples:

• gvidinta = having guided / having led
• alveninta = having arrived

When used with esti or estus, it helps form a completed-action meaning:

• estus gvidinta = would have guided
• estus alvenintaj = had arrived / would have arrived, depending on context

Because these participles behave like adjectives, so they agree with the subject.

• amikino is singular, so: gvidinta
• ni is plural, so: alvenintaj

Compare:

• Mi estus alveninta = I would have arrived
• Ni estus alvenintaj = We would have arrived

So the -j is there because ni is plural.

Because nin is the direct object of gvidi.

Ask the question:

• She would have guided whom?
  → us

So Esperanto marks that with -n:

• ni = we
• nin = us

Because it comes after the preposition preter.

In Esperanto, nouns after prepositions usually do not take the accusative just because there is movement.

So:

• preter la malnova kvartalo = past the old quarter

not:

• preter la malnovan kvartalon

The accusative after a preposition is mainly used in certain directional expressions with place-prepositions, such as:

• en la domo = in the house
• en la domon = into the house

But preter already contains the idea of movement past something, so no extra -n is needed on kvartalo.

Preter means past or by, in the sense of moving alongside something and going beyond it.

So:

• gvidi nin preter la malnova kvartalo
  = guide us past the old quarter

It suggests a route that goes past that area, not necessarily through it.

Useful comparisons:

• preter = past
• tra = through
• apud = beside / next to

Amikino means female friend.

It is built from:

• amik- = friend
• -in- = female
• -o = noun ending

So:

• amiko = friend / male friend
• amikino = female friend

And mia amikino means my female friend.

Kvartalo means district, neighborhood, or quarter of a town or city.

So:

• la malnova kvartalo = the old quarter / the old neighborhood

In this sentence, it most likely refers to a particular part of town, probably an older or historic area.

Yes, it is completely normal.

• frue = early
• pli frue = earlier

Literally, it is more early, just as Esperanto regularly forms comparatives with pli.

So:

• Ni alvenis frue = We arrived early
• Ni alvenis pli frue = We arrived earlier

In this sentence:

• se ni estus alvenintaj pli frue
  = if we had arrived earlier

It has two parts:

1. Main clause:
   Mia amikino estus gvidinta nin preter la malnova kvartalo
   = My female friend would have guided us past the old quarter

2. Condition clause:
   se ni estus alvenintaj pli frue
   = if we had arrived earlier

So the whole sentence is a past unreal conditional:

• If X had happened, Y would have happened.

In other words:

• we did not arrive earlier
• therefore, she did not guide us past the old quarter