Preposition + Infinitive: The One-Subject Shortcut

Preface: al Has Two Different Jobs

1️⃣ al = a + el (to / toward / into)

This is the everyday contraction of the preposition a + the masculine article el.

• Voy al parque. → to the park
• Subir al metrobús. → onto the bus
• Dar algo al niño. → to the boy
  

It’s pure direction or indirect-object a.

2️⃣ al + infinitive = a + el (upon / when)

Here the el is not a definite article—it’s the article for a verbal noun.

Verbal Noun: Spanish treats the infinitive (llegar, salir, entrar) as a noun (“the arriving,” “the leaving”), so a + el llegar literally means “at the arrival.”

• Al llegar a casa → At the moment of arriving home / Upon arriving home
• Al escuchar la noticia → Upon hearing the news

It’s using the same contraction (a + el), but here el is functioning like “the [act of] arriving,” not “the [masculine article] park.”

Al + infinitive

In everyday Mexican Spanish, the “al + infinitive” construction is very common and sounds completely natural.

It literally means:

• “upon ___-ing” or 
• “when ___ happens” 

And is used to express that something occurs at the moment of or right after another action.

You could say: When I arrived home, I went to bed.
• Cuando llegué a casa, me acosté. – which means you have to remember to conjugate llegar in the first clause.
Or you could say: Upon arriving home, I went to bed.
• Al llegar a casa, me acosté.
They both mean the same thing, but you don’t have to conjugate the (al) version.

You could say: When I finished the report, I sent it to my boss.
• Cuando terminé el informe, se lo envié al jefe.
Or you could say:  Upon finishing the report, I sent it to my boss.
• Al terminar el informe, se lo envié al jefe. 
Note: the second al in clause two, is the a + el [masculine article] contraction.

You could say: When I saw my friend, I waved at him.
• Cuando vi a mi amigo, lo saludé.
Or you could say:  Upon seeing my friend, I waved at him.
• Al ver a mi amigo, lo saludé.

Note: Even though the English equivalent uses a present participle (“arriving,” “finishing”), Spanish always uses the infinitive form after al.

Antes De + infinitive

–  before doing something

Use this only when the same subject performs both actions.
Because the verb after antes de stays in the infinitive, it implies the subject is the same as the main clause unless you explicitly say otherwise.

Antes de salir, compré pan.
(Before leaving, I bought bread. — I left and I bought.)

If the two actions have different subjects, switch to antes de que + subjunctive.

Antes de que tú llegaras, yo ya había preparado todo.
(Before you arrived, I had already prepared everything. — different people.)

Antes de salir, compré pan. – Before leaving, I bought bread. (one subject doing both things, me)

Antes de acostarme, vi una película. – Before going to bed, I watched a movie. (one subject doing both things, me)

Antes de empezar la película, ella apagó su teléfono. – → Before starting the movie, she turned off her phone. (one subject doing both things, she)

Antes de viajar a Oaxaca, reservamos el hotel.- → Before traveling to Oaxaca, we booked the hotel. (one subject, we)

Antes de entrar al estadio, ellos compraron comida. – → Before entering the stadium, they bought food. (one subject, they)

Después de + infinitive 

– After doing something

después de + infinitive follows the exact same pattern as antes de + infinitive.
Use when the same person does both actions or when time is involved (future or past action).

Después de comer, salimos a caminar. – (After eating, we went for a walk. – we ate and we walked)

Después de terminar la tarea, ella vio la tele. –  (After finishing the homework, she watched TV. – she did both)

Después de ver la película, me acosté. – After watching the movie, I went to

Bed.

En vez de + infinitive / En lugar de + infinitive 

– Instead of doing something

Use when one person does (or chooses not to do) the action.
Two subjects will trigger a subjunctive sentence.

En vez de trabajar, decidí descansar. – Instead of working, I decided to rest.

En lugar de estudiar, fui a la playa. – Instead of studying, I went to the beach.

En lugar de tomar café, tomo té. – Instead of drinking coffee, I drink tea.

En vez de mirar la tele, él salió a caminar. – Instead of watching TV, he went for a walk.

Sin + infinitive 

– Without doing something

Same subject → sin + infinitive (no conjugation).
Different subjects → sin que + subjunctive.

Ella se fue sin despedirse. – She left without saying goodbye.

Me comí la pizza sin calentarla. – I ate the pizza without heating it.

Salí sin desayunar. – I left without eating breakfast.

Se fue sin despedirse. – She left without saying goodbye.

Para + infinitive 

– In order to do something

Same subject → para + infinitive
Different subjects → para que + subjunctive

Salimos temprano para llegar a tiempo. – We left early in order to arrive on time.

Compré flores para decorar la casa. – I bought flowers to decorate the house.

Estudio español para viajar a México. – I study Spanish in order to travel to Mexico.

Traje mi computadora para trabajar en el café. – I brought my laptop to work at the café.

Salimos temprano para evitar el tráfico. – We left early to avoid the traffic.

Reflexive Verbs and Pronoun Placement

The reflexive pronoun attaches to the infinitive only in that first clause (the preposition + infinitive part).
The second clause, if it has a verb, follows normal conjugation rules.

Breakdown

• Antes de acostarme, vi una película.
  acostarme → pronoun (me) sticks to the infinitive acostar.
  vi → regular past-tense verb.
  
• Antes de acostarte, cepíllate los dientes.
  acostarte → pronoun (te) stays on the infinitive.
  cepíllate → imperative with its own pronoun.
  

If the second clause also uses a reflexive verb, it behaves as usual:

Antes de acostarme, me lavé la cara.
(first me attached to acostar, second me as a normal pre-verb reflexive for lavé).

These constructions offer an efficient way to speak Spanish without pausing to conjugate

multiple verbs. They are especially useful for narrating events of one person or group (they).