"I cannot write this for you."
Translation:Nie mogę napisać tego za ciebie.
I don't know much about the rules, I just feel the language, as it is my own - I'd say that it's perfective (in Polish we use the distinction dokonany/niedokonany, so kinda 'accomplished/not accomplished') is simply used because the interlocutor most probably wanted me to write the whole thing. "pisać" would focus on the process of writing.
is it not a situation borderline? If I say this sentence, for example, during an exam, with the meaning of a forbidden action because the prof could catch me, perfective wouldn't be more appropriate?
But we already have perfective here.
In most sentences, both are unfortunately technically possible, but usually one is waaaaaay more probable.
Cases. 'pisać' and 'napisać' take Accusative, so here where it's negated (okay, technically 'mogę' is negated, but it's like the whole sentence is negated), it then takes Genitive. Which is "tego".
Nie mogę napisać tego dla ciebie - I can't write this for you.
Nie mogę napisać tego za ciebie - I can't write this for you (instead of you), you'll have to do it yourself.
Does english sentence mean only "for" in sense that "this is for you" or also "instead of"?
Dla can be only used when translating it as first option and za when translating it as "instead of"?
yes, in english "I can do this for you" could mean as a favor, or a service, or a gift for you, or it can mean instead of you, or in your place (so you don't have to do it yourself)... I think in spoken language the meaning would be more clear depending on which word you stress in the sentence "I can DO this for you" (as in, "I'm willing and able to do this for you) or I can do this FOR you; or the stress on "I" can do this for you (as in, so you don't have to do it, or "here, let me do it instead of/for you") and I guess in Polish they have two different words for the different meanings :)
Dla in the first context and za in the second.
btw why is "nie mogę tego za ciebie napisać" accepted but "nie mogę tego dla ciebie napisać" isn't?
If I wanted to alter the sentence "Nie mogę napisać tego za was" to use an unstressed form of "ty" instead of "was", could I do that? What would it look like? It's not liking "Nie mogę tego za cię napisać".
It's not wrong, it's just not the first thing that comes to your mind when you see 'for you'.