Mi hermana olvidó validar el abono al salir y una empleada la esperó junto al torniquete.

In Spain, abono very often means a travel pass / season ticket / transport subscription, especially in metro, train, or bus contexts.

So here it does not mean things like:

• fertilizer
• payment
• subscription in a more general sense

In this sentence, it refers to the kind of pass you use on public transport and may need to scan or validate.

In Spain, validar is the normal transport word for validating / scanning / tapping a ticket or pass at a machine or gate.

So validar el abono means something like:

• using the pass at the reader
• registering it properly
• tapping it when entering or leaving, depending on the system

Even if the pass is already paid for, you may still need to validate it so the journey is officially recorded.

Both are possible.

• olvidar + infinitive = to forget to do something
• olvidarse de + infinitive = also to forget to do something

So:

• olvidó validar
• se olvidó de validar

can both work here.

The version in your sentence is a little more direct and compact. The olvidarse de version is also very common in everyday speech.

Al + infinitive is a very common Spanish structure meaning:

• when doing
• upon doing
• on doing

So al salir means:

• when leaving
• on the way out
• upon exiting

Grammatically, al is the contraction of a + el, but in this pattern you should mostly learn it as a fixed structure:

• al entrar = when entering
• al llegar = when arriving
• al salir = when leaving

They are in the preterite because the sentence is describing a specific completed event in the past.

• olvidó = she forgot
• esperó = she waited for / stayed waiting for

This sounds like a one-time incident, not a habit or background description.

If the sentence were describing ongoing background or repeated behavior, Spanish might use the imperfect instead, but here the preterite is the natural choice.

La is a direct object pronoun referring back to mi hermana.

• mi hermana is feminine singular
• so the pronoun is la

In English, we might repeat the noun less naturally:

• the employee waited for my sister

In Spanish, once mi hermana has already been mentioned, using la is very natural:

• una empleada la esperó

It means an employee waited for her.

Because esperar takes a direct object, and the standard direct object pronoun for a feminine singular person is la.

So:

• la esperó = she waited for her

This is standard Spanish.

You may have heard of leísmo in Spain, where le is sometimes used instead of lo for masculine people. But that usually does not apply here with a feminine object. For mi hermana, la is the expected form.

Good question. With a full noun, Spanish would normally use the personal a:

• una empleada esperó a mi hermana

But when you replace that noun with a direct object pronoun, you do not use the a:

• una empleada la esperó

So:

• esperó a mi hermana
• la esperó

are parallel forms.

Because this is the first time that employee is being mentioned.

• una empleada = an employee, some employee
• la empleada = the employee, a specific one already known or already mentioned

So the indefinite article una introduces a new person into the story.

Also, empleada is feminine because the worker is female. If it were a male worker, it would be un empleado.

Junto a means next to, beside, or right by.

So:

• junto a + el torniquete becomes junto al torniquete

This happens because a + el = al.

So the structure is:

• junto a = next to
• el torniquete = the turnstile
• junto al torniquete = next to the turnstile

This is very similar to al lado del torniquete, though junto a can sound a bit more direct and compact.

Spanish often leaves out subject pronouns because the verb forms and context already make the subjects clear.

In this sentence:

• Mi hermana olvidó... clearly gives the subject of the first clause
• una empleada la esperó... clearly gives the subject of the second clause

So adding ella would usually be unnecessary unless you wanted special emphasis or contrast.

This omission of subject pronouns is one of the most common features of natural Spanish.