Why Portuguese says 'roubar a alguém' for 'steal from someone'
In English, you steal, buy or borrow something from a person. Portuguese often sees it differently. The person who loses the item, or who hands it over, is treated as someone affected by the action, so they are introduced with a (to) rather than de (from). Roubaram o carro ao meu vizinho means "They stole the car from my neighbour."
This trips up English speakers because a looks like it should mean "to", so the sentence seems to say the car went to the neighbour. The same pattern appears with comprar, furtar, tirar, pedir emprestado and even with hiding verbs like ocultar. It is especially typical of European Portuguese.
The basic shape is verb + something + a + person. The thing taken is the direct object, and the person who ends up without it is an indirect object. Remember that a merges with the article: a + o = ao and a + a = à.
Because the person is an indirect object, a pronoun replaces them with the indirect forms me, te, lhe, nos, lhes. This is the version you will hear most often in everyday speech.
Rule of thumb: if English says "from someone" and that person loses or gives up something, try a or an indirect pronoun.
Roubar and furtar both work this way. Furtar suggests quiet theft without violence, like pickpocketing.
Tirar (to take out, to remove) and retirar (more formal) use a for the person who loses the item.
With comprar and pedir emprestado, the person is not a victim but the source who hands something over. The grammar is the same.
Keeping something from someone follows the same logic: that person is deprived of the information. Ocultar works like this, and so does the more everyday esconder.
In Brazilian Portuguese, comprar de alguém and pedir emprestado de alguém are very common, and theft is often expressed with a possessive instead.
Roubar algo a alguém puts the stolen item first. When the person is the direct object, as in Brazilian roubaram o turista, the meaning is "they robbed the tourist". In Portugal, assaltar is the usual verb for that.
Comprei uma prenda ao João usually means "I bought a present for João". Context tells you whether João is the seller or the one receiving the gift.
When you hide yourself, use esconder-se de alguém, with de.
When someone loses, hands over or is kept from something, European Portuguese usually marks that person with a (often ao or à) or with an indirect pronoun like me or lhe: roubaram-me a carteira, comprei o livro ao professor. Think "done to someone", not "from someone", and expect de more often in Brazil.