Ayer choqué con mi vecino al salir del ascensor.

Because the verb is chocar, and verbs ending in -car make a spelling change in the yo form of the preterite:

• chocar → choqué
• buscar → busqué
• sacar → saqué

The c → qu change happens before é to keep the hard k sound. If you wrote chocé, it would sound different.

It is in the preterite because the sentence describes a single completed event in the past: yesterday I bumped into my neighbor.

Spanish often uses the preterite for:

• completed actions
• specific events
• actions tied to a finished time expression such as ayer

So Ayer choqué... is the natural choice.

Spanish often leaves out subject pronouns when they are clear from the verb ending. The ending -é in choqué already tells you the subject is I.

So:

• Ayer choqué con mi vecino...
• Ayer yo choqué con mi vecino...

Both are possible, but the version without yo is more normal unless you want extra emphasis.

Here chocar con means to bump into or to collide with someone or something physically.

So choqué con mi vecino means that there was some kind of physical contact or collision.

Depending on context, chocar con can also be used more figuratively, for example for ideas or personalities that clash, but in this sentence the physical meaning is the natural one.

Because chocar commonly takes the preposition con when you say what or who you collided with:

• chocar con alguien
• chocar con algo

So:

• Choqué con mi vecino
• El coche chocó con un árbol

This is just the normal Spanish pattern, even though English may use different phrasing depending on the context.

Al + infinitive is a very common Spanish structure meaning when doing, upon doing, or as someone does something.

So al salir means:

• when leaving
• as I was leaving
• upon leaving

In this sentence, al salir del ascensor tells you the moment when the collision happened.

Both are possible, but they are slightly different in style.

• al salir del ascensor = when leaving / upon leaving the elevator
• cuando salí del ascensor = when I left the elevator

Al + infinitive is compact and very natural when one action happens at the moment of another. It often sounds smoother in sentences like this.

Also, with al salir, the subject is normally understood to be the same as in the main verb, so it naturally means when I was leaving the elevator.

Because de + el contracts to del in Spanish.

So:

• de el ascensor → del ascensor

This contraction is mandatory.

Compare:

• voy al cine = a + el
• salgo del ascensor = de + el

The only major exception is when El is part of a proper name, such as de El Escorial.

Yes. In Spain, ascensor is the normal word for elevator.

A learner should also know:

• In British English, this is lift
• In much of Latin America, elevador is very common

But in Spain, ascensor is exactly the word you would expect here.

Yes. Spanish word order is flexible, so you could move ayer:

• Ayer choqué con mi vecino al salir del ascensor.
• Choqué con mi vecino ayer al salir del ascensor.
• Choqué ayer con mi vecino al salir del ascensor.

The first version is very natural because it sets the time immediately. Putting ayer first is a common way to frame the whole sentence.

Grammatically, vecino is masculine singular, so it refers to a male neighbor.

If the neighbor were female, you would say:

• mi vecina

If you do not know the person’s gender, Spanish usually still has to choose a grammatical form, so the sentence as written specifically points to a male neighbor.

Vecino usually means a person who lives nearby, especially in the same building, on the same street, or in the same area. It does not have to mean the person literally in the apartment right next to yours.

So mi vecino could be:

• the person next door
• someone else in the building
• someone nearby in the neighborhood

The exact relationship depends on context.

By default, Spanish readers understand it to be the same person as the main subject of choqué, so the natural reading is:

• I bumped into my neighbor when I was leaving the elevator

That is a very common feature of al + infinitive. Unless context clearly says otherwise, the subject is usually the same as the subject of the main clause.