I decide something by myself. = "Eu decido algo por mim mesma." or "Eu decido algo sozinha." (It's not usual to say "por mim próprio" in brazilian portuguese)
I can repair this by myself. = "Eu mesma posso consertar isso." or "Eu posso consertar isso sozinha." (It's also possible "Eu posso consertar isso por mim mesma.")
I will go to Portugal by myself. = Eu irei a Portugal sozinha.
Guys how can I use by myself form in European Portuguese ?
Can you give me some examples for me to understand.
The expression "by myself" can be translated as "por mim mesmo/mesma" or "sozinho/sozinha" or even "só". You'll have to learn the situations where each translation fits better.
Don't wanna be all by myself. = Não quero ficar totalmente sozinha.
I sat at the table by myself. = Eu sentei à mesa sozinho.
I always go for a walk by myself. = Eu sempre caminho sozinho.
I will not be at the party. I will be at home by myself tonight. = Eu não estarei na festa. Estarei em casa sozinho hoje à noite.
Pay attention because there are other english expressions that could be used to be translated as sozinho, like "on my own" or the adverb/adjective "alone".
Eu construí o armário sozinho. = "I built the wardrobe by myself." "I built the wardrobe alone."
I guess you can use "by myself" and "all by myself" and it will have the same effect on the translation to portuguese. Also I think using just "myself" will be most usually translated by "mesmo/mesma"
I did it myself. = "Eu mesmo fiz."
I did it by myself. = "Eu fiz sozinho."
I'd advise you to translate "by myself" as "sozinha". You'll also understand with time that some other expressions share the semantics in the most different situations. "por mim mesma", "mesma", "própria"
Good luck with your studies!