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Prepositions and the personal infinitive

In English, when a verb follows a preposition, it almost always takes the -ing form. We say "before leaving" or "without knowing". When learning Portuguese, English speakers often want to use the Portuguese gerund to translate these phrases. However, Portuguese works differently: after prepositions, you must use the infinitive.

Portuguese takes this a step further with a unique grammar feature called the personal infinitive. This means you can conjugate the infinitive to match the subject of the action. This happens after everyday prepositions like antes de, depois de, and the very common word para.

Word list
antes deA1 before antes de sairmos atéA1 until até chegares depois deA1 after depois de comermos semA1 without sem eles saberem ao invés deA2 instead of ao invés de apósA2 after após terminarmos em vez deA2 instead of em vez de esperarmos além deB1 beyond além de sabermos apesar deB1 despite apesar de estarem cansados em lugar deB1 instead of em lugar de sairmos graças aB1 because of graças a termos estudado porB1 because of por estarem cansados por causa deB1 because of por causa de chegarmos tarde devido aB2 because of devido a estarem ocupados não obstanteB2 despite não obstante estarem cansados por conta deB2 because of por conta de terem chovido tanto a despeito deC1 despite a despeito de
How the personal infinitive works

When a preposition is followed by an action, you use the infinitive. If the person doing the action is "I" or "he, she, it", the personal infinitive looks exactly like the dictionary form of the verb. But if the subject is plural (like "we" or "they") or the informal "you", you add an ending directly to the infinitive.

For example, for nós, you add -mos. For eles, elas, or vocês, you add -em. For tu, you add -es.

Vamos comer antes de sairmos.
(Let's eat before we leave.)
Lavo a louça depois de eles comerem.
(I wash the dishes after they eat.)

You can use the personal infinitive whether the action shares a subject with the main sentence or has a completely different subject. It makes it crystal clear who is performing the action.

Eles saíram sem nós sabermos.
(They left without us knowing.)
Vou esperar aqui até tu chegares.
(I will wait here until you arrive.)
Cause, effect, and contrast

Several prepositions that introduce reasons or contrasting ideas also call for the personal infinitive. The word por translates to "because of" in this context. To express "despite", you can use apesar de or the more formal a despeito de.

As crianças ficaram em casa por estarem doentes.
(The children stayed home because they were sick.)
Fomos à praia apesar de chover.
(We went to the beach despite it raining.)

In the second example, chover is in the regular infinitive form because the weather does not have a plural subject.

Expressing alternatives

When you want to say that someone did one thing instead of another, you have a few options. The most common phrases are em vez de and em lugar de. These can be used for any kind of substitution.

Bebemos chá em vez de tomarmos café.
(We drank tea instead of having coffee.)

You might also see ao invés de. Careful writers prefer to use this specifically when the two actions are exact opposites, rather than just random alternatives.

Ele acelerou ao invés de travar.
(He accelerated instead of braking.)
Summary

When an action follows a preposition in Portuguese, always use the infinitive instead of a gerund. If the person performing the action is clear, especially if the subject is plural or the informal "you", conjugate the infinitive by adding the correct ending to the dictionary form of the verb.