Perhaps I should I ask my question somewhere else, but I am totally at a loss. Why Eu penso em ti, but Eu falo de ti. Both em and de mean the same - about? Can I then say Eu penso de ti and Eu falo em ti. Any difference between any of these? Thank you.
Olá, @annaybeck 
Prepositions work differently in Portuguese and in English, and the same preposition can have multiple English translations depending on context. Different prepositions may also be translated similarly without actually being interchangeable in Portuguese → one of the possible reasons for this is that a certain verb asks specifically for a certain preposition.
Eu penso em ti
This is an example of a verb that requires a specific preposition. When used to describe the general act of thinking about something, the verb pensar can only be paired with the preposition em. So, Eu penso de ti is not a correct sentence. You can only use pensar de when you want to talk specifically about what you think of something. Some examples to clarify:
- Tu pensas em mim? (Do you think about me?)
- Penso em fazer isso um dia. (I think about doing that one day)
- Pensei no que ela disse. (I thought about what she told me - remember that no = em+o)
- Não sei o que pensas de mim. (I don’t know what you think of me.)
- Queres saber o que eu penso dele? (Do you want to know what I think of him?)
In each of the examples above, the preposition given is the only one that’s acceptable. There’s no interchangeability.
Eu falo de ti
With the verb falar, things are more flexible, because the verb accepts multiple prepositions. Generally speaking, you can use falar de, falar em and falar sobre interchangeably, but with some nuances that are discussed in this other topic: Falar de ou falar em? - Portuguese Questions - Practice Portuguese
So, all examples below are grammatically correct:
- Eu falo de ti.
- Eu falo em ti.
- Eu falo sobre ti.
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Thank you so much for this, Joseph! Most helpful.
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