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:
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:
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:
So:
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.
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 + 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.
Yes. Spanish word order is flexible, so you could move ayer:
The first version is very natural because it sets the time immediately. Putting ayer first is a common way to frame the whole sentence.
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 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:
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.