Infinitive or gerund: al llegar, de haberlo sabido

English -ing has many jobs: noun (Smoking kills), after prepositions (before leaving), manner (by working), progressive (is working). Spanish splits these between the infinitive (-ar, -er, -ir) and the gerund (-ando, -iendo, see Estar + gerund: what is happening now). Getting this right is a clear marker of B2 Spanish.

The infinitive as a noun

When an action is the subject or object, Spanish uses the infinitive, never the gerund.

• Leer en el móvil me cansa la vista.Reading on my phone strains my eyes.

After a preposition: always the infinitive

Antes de, después de, sin, en vez de, para, por, tras + infinitive.

• Se fue sin despedirse de nadie.He left without saying goodbye to anyone.
• En vez de quejarte, propón una solución.Instead of complaining, suggest a solution.

For an earlier action, use the perfect infinitive (haber + participle):

• Tras haber firmado el acuerdo, los dos líderes dieron una rueda de prensa.After signing the agreement, the two leaders gave a press conference.

Al + infinitive: when, on, as

Al + infinitive means when / as / on doing. The subject can be the same as in the main clause or different.

• Al salir del metro, me encontré con mi antigua jefa.As I came out of the metro, I ran into my former boss.

It can also give a cause, especially in the negative: Al no tener experiencia, no me contrataron (As I had no experience…).

De + infinitive: a condition

De + infinitive replaces a si-clause (Hypotheses: si tuviera, iría, Past hypotheses: si hubiera sabido, habría venido). It is common and slightly formal.

• de + simple infinitive = si + present or imperfect subjunctive
• de + haber + participle = si + pluperfect subjunctive

• De seguir así, la empresa cerrará antes del verano.If things carry on like this, the company will close before the summer.
• De haberlo sabido, habría reservado otro hotel.Had I known, I'd have booked another hotel.

The gerund: how, by what means, at the same time

The gerund describes the manner or means of the main action, or something happening at the same time:

• Aprendí a programar viendo tutoriales.I learned to code by watching tutorials.
• Se gana la vida traduciendo novelas.She makes a living translating novels.

It can also give a reason at the start of a sentence:

• Siendo domingo, el banco estará cerrado.Since it's Sunday, the bank will be closed.

And it appears in periphrases: seguir + gerund (keep doing), ir + gerund (gradually), llevar + gerund (have been doing, How long: hace … que, desde hace, llevar + gerund).

• La situación va mejorando poco a poco.The situation is gradually getting better.

Where the gerund is wrong

• Not as an adjective after a noun: una caja que contiene libros, not una caja conteniendo libros (a calque from English, sometimes called the "gerund of the official bulletin").
• Not for a later, separate consequence: prefer Se cayó y se rompió la pierna to Se cayó rompiéndose la pierna (the RAE advises against this "gerund of posteriority").

After verbs of perception both forms work: La vi salir (I saw her leave) and La vi saliendo (I saw her leaving).

Common mistakes

• Fumando es malo: an action as subject is the infinitive: Fumar es malo.
• antes de saliendo: after a preposition, always the infinitive: antes de salir.
• Recibí una carta conteniendo: use a relative clause: una carta que contenía.
• En llegando a casa is old-fashioned or regional. Say Al llegar a casa.
• Por trabajando for "by working": the gerund alone carries "by": trabajando.

Latin America

The rules are the same. Al + infinitive and de + infinitive are used everywhere; in many places the de + infinitive condition sounds a little formal or literary in casual conversation, where a si clause is more usual.