I know you use a + infinitive for forming other tenses like “ing” etc. But I came across the sentence “o cão aprende a saltar” meaning: The dog learns to jump - and had a Question.
Why do we need the ‘a’ before saltar because as an infinitive, doesn’t saltar translate to: to jump. What extra value does the ‘a’ add?
My understanding (which may be wrong) is that the ‘a’ is often used but not obligatory. So “o cão aprende saltar” would have been equally correct.
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@rachelbenmenachem, in Portuguese, infinitives have no implied preposition in them. So, if a preposition is required, it still needs to be added before the infinitive. The English translation of an infinitive normally includes ‘to’ simply as an adaptation to English grammar 
Apart from cases where the a + infinitive expresses continuity (e.g. estar a fazer = to be doing), there are also cases where we have transitive verbs that require a linking preposition whenever the object is in the infinitive. That’s what happens here with aprender.
- Aprendi matemática na escola. (I learned maths in school → no infinitive, no preposition)
- Aprendi a fazer cálculos na escola. (I learned [how] to make calculations in school → preposition before the infinitive)
An example with another verb:
- Ensino dança às terças. (I teach dance on Tuesdays)
- Ensino a dançar às terças. (I teach [how] to dance on Tuesdays)
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Just to understand it correctly @Joseph , are you saying that all transitive verbs need a preposition if the object is an infinitive, or only that some do? If it’s only some, is there an easy way to know, or is down to memorising them?
Some verbs need a preposition in these cases. Unfortunately, I would say there’s no way around simply memorizing them as you come across them.
For example:
- Tu precisas de comer. (You need to eat)
- Eu gosto de trabalhar. (I like to work)
But: - Ela quer viver em Coimbra. (She wants to live in Coimbra)
- Nós pretendemos fazer isto. (We intend to do this)
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