Learning Italian with a base of Spanish and Portuguese
What are the most important differences I should nail to make a good transition from these languages (and maybe also French) to Italian?
I will answer this question from my personal perspective. I don't speak portuguese but I'm a native spanish speaker and I spent a while learning Italian. First of all, I'd like to start saying that you are in advantage, because Spanish, Portuguese and Italian are very alike. Without even taking a single lesson of portuguese and italian, a spanish speaker can understand a reasonable amount of what is being said in a conversation. I even had a conversation with someone from Brazil, he was speaking portuguese and I was speaking spanish... I can tell you IT WASN'T PERFECT but we could understand each other. Crazy huh?
Well, to make this transition I think you should focus on pronunciation and grammar. Don't get me wrong, the vocabulary of the language is very important but you shouldn't worry too much about that, since you are going to understand a great amount of words without even making an effort. I'm going to give you some examples:
Window = Finestra (Italian), Fenetre (French)
To do = Fare (Italian), Faire (French)
To eat = Mangiare (Italian), Manger (French)
To Sing = Cantare (Italian), Cantar (Spanish)
These are just a few examples, there's a lot more of course :)
The grammatic aspect of the languages will not bother you (too much, there's always exceptions hehe), since these three languages are very similar in that aspect. Oh, and before I forget to tell you, be careful with "les faux amis" or "falsi amici", since they can really change a sentence's sense. A few exemples of these are:
Burro = In spanish it means DONKEY, while in Italian it means BUTTER.
Bruto = In spanish it means "stupid", while in italian it means "ugly". (By the way, in italian it's spelled with a double t, but some people can't differenciate between them.
If you have any more questions or want me to give more detail I'm glad to help. I hope I made myself clear (My english is not perfect). Have a nice day :D