I think it could be interesting to know, how the Spanish suffixes/endings (or maybe also prefixes?) change in Portuguese, as it could help a lot to learn one of these if you already know the other.
I know that one rule is
and I think the ending -tiva stays -tiva (example: expectativa - expectativa - the same word in both languages, but with different pronunciation)
Does anyone know more rules concerning these 2 languages?
Thanks! Yes, that is exactly what I meant. I didn’t know that Wikipedia dedicates a whole website about “comparison of Spanish and Portuguese”. I only typed words like “different endings” or “suffixes” + the 2 languages, but I didn’t get any satisfactory results via Google.
There’s also an FSI manual that is available in PDF form online that teaches Spanish speakers how to use their Spanish in order to learn Portuguese - a must have!
Thanks for the hint! I’ve just downloaded this pdf, but I only see English translations of the Portuguese words (no ESP-P/Br comparisons).
But I think it could be, nevertheless, helpful to enlarge my vocabulary.
I not only realized that a prior knowledge of Spanish is very helpful to learn Portuguese, but it can - in a very few cases - make it more difficult, too. This is the case, when 2 complete same words have a complete different meaning (the well known “false friends”).
I can - for example - not really get accustomed to the fact, that “acordar” means “to wake up” in Portuguese. This is very confusing. 
There definitely is a whole section on just that… The manual is over 100 pages long. It talks about how ll words in Spanish are typically ch words in portuguese… ue dipthongs in Spanish are typically o words in Portuguese… llano = chão … fuego = fogo … puerto = porto, etc. tons more examples…
I agree the false friends can throw you at first, but there are so many more ¨real friends¨ than false ones that learning the exceptions is certainly an easier job than learning from scratch…