La policistino, kiu antaŭe igis nin halti ĉe la semaforo, poste eniris la saman librovendejon.

Policistino is built like this:

• polic- = police
• -ist- = a person professionally associated with something
• -in- = female
• -o = noun ending

So:

• policisto = police officer / policeman
• policistino = female police officer / policewoman

Esperanto often uses -in- to mark female nouns:

• patro = father/parental father
• patrino = mother
• instruisto = teacher
• instruistino = female teacher

La is the definite article, meaning the.

So La policistino means the policewoman / the female police officer, not just a policewoman.

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

• la viro = the man
• la virino = the woman
• la libroj = the books

Here kiu means who, introducing a relative clause:

• La policistino, kiu antaŭe igis nin halti...
• The policewoman, who earlier made us stop...

So kiu refers back to la policistino.

In Esperanto, kiu can mean:

• who
• which
• that
  depending on context.

Because policistino is singular, kiu is singular too.

That part is a relative clause giving extra information about the policewoman.

The commas show it is a non-restrictive relative clause: the speaker already has a specific policewoman in mind, and the clause just adds information about her.

So the structure is:

• main clause: La policistino ... poste eniris la saman librovendejon.
• inserted extra information: kiu antaŭe igis nin halti ĉe la semaforo

This works much like English:

• The policewoman, who had earlier made us stop at the traffic light, later entered the same bookstore.

Antaŭe is an adverb meaning earlier, beforehand, or previously.

• antaŭ usually works as a preposition meaning before in space or time:

  • antaŭ la domo = in front of the house
  • antaŭ la vespermanĝo = before dinner

• antaŭe is the adverb:

  • Mi antaŭe vidis lin. = I saw him earlier.

In your sentence, antaŭe means the stopping happened earlier than the bookstore action.

This is a very common Esperanto pattern:

• igi = to cause, make, cause to become/do
• halti = to stop

So:

• igis nin halti = made us stop / caused us to stop

This is the pattern:

• igi + object + infinitive

Examples:

• Ŝi igis min ridi. = She made me laugh.
• La bruo igis la bebon vekiĝi. = The noise made the baby wake up.

So here:

• la policistino = the person causing the action
• nin = the people being caused
• halti = the action they are caused to do

Because nin is the direct object form of ni.

• ni = we
• nin = us

In igis nin halti, the policewoman made us stop, so us is the object of igis.

Compare:

• Ni haltis. = We stopped.
• Ŝi igis nin halti. = She made us stop.

The -n marks the accusative, which often shows the direct object.

Because halti means to stop, and igi already provides the idea of making/causing.

So:

• igi + halti = make stop / cause to stop

If you used haltigi, that verb itself already means to stop something/someone or to cause to stop.

So these are close in meaning:

• Ŝi igis nin halti.
• Ŝi haltigis nin.

Both can mean She stopped us / She made us stop.

The version in your sentence is a very transparent, beginner-friendly causative structure.

Ĉe usually means at, by, with, or in the vicinity of, depending on context.

So:

• ĉe la semaforo = at the traffic light

Semaforo means traffic light or signal.

Why ĉe? Because the idea is location near a point:

• ĉe la pordo = at the door
• ĉe la stacidomo = at the station
• ĉe la semaforo = at the traffic light

It does not mean being physically inside something. It means being at that location.

Poste is an adverb meaning later, afterward, or then.

It contrasts nicely with antaŭe:

• antaŭe = earlier, previously
• poste = later, afterward

So the sentence gives a sequence:

1. earlier, she made us stop at the traffic light
2. later, she entered the same bookstore

Both patterns exist, but they are slightly different in style.

• eniri = to enter
• iri en = to go into

So:

• Ŝi eniris la vendejon. = She entered the shop.
• Ŝi iris en la vendejon. = She went into the shop.

Eniri is a single verb built from:

• en- = in, into
• iri = to go

It is very common and often sounds a bit more compact and natural than iri en.

Because eniris takes a direct object here, and the whole noun phrase is in the accusative.

• la sama librovendejo = the same bookstore
• la saman librovendejon = the same bookstore, as a direct object

In Esperanto, adjectives agree with nouns in:

• number
• case

So if the noun gets -n, the adjective does too:

• la granda domo
• la grandan domon

Here:

• sama becomes saman
• librovendejo becomes librovendejon

In Esperanto, sama is very commonly used with la to mean the same:

• la sama libro = the same book
• la sama loko = the same place

So:

• la saman librovendejon = the same bookstore

Without la, sama often sounds less natural or changes the nuance.

It is a classic Esperanto compound:

• libr- = book
• vend- = sell
• -ej- = place for an activity
• -o = noun

So:

• librovendejo = a place for selling books = bookstore / bookshop

Related words:

• vendi = to sell
• vendejo = shop, store, sales place
• librobutiko can also be used for bookstore, but librovendejo is very transparent

Yes, depending on the exact nuance.

• la saman librovendejon = the same bookstore
• tiun saman librovendejon = that same bookstore
• tiun librovendejon = that bookstore

Using la saman focuses on sameness: it was the very same bookstore as one already known from context.

Using tiun saman would point to it a bit more explicitly, as in that very same bookstore.

So the original sentence is perfectly natural, and la saman is a very standard way to say the same.

The skeleton is:

• La policistino ... poste eniris la saman librovendejon.
• The policewoman ... later entered the same bookstore.

Inside that, there is an inserted relative clause:

• kiu antaŭe igis nin halti ĉe la semaforo

So the full structure is:

1. subject: La policistino
2. relative clause describing the subject: kiu antaŭe igis nin halti ĉe la semaforo
3. main verb: eniris
4. object: la saman librovendejon

This kind of sentence is very common in Esperanto, and once you spot the relative clause, the grammar becomes much easier to follow.