Help to write pa svenska

Well.... there are some problems with all of those, because they might not convey the meaning you're after.

"Han knackar dörr" means "he goes door to door", as in someone who's trying to sell something. Like a travelling salesman, or a Jehova's Witness.

"Han knackar på dörr" would translate to "he knocks on door". That's just not correct. It might work, claiming poetic license, but a Swedish speaker would just think it's weird and wrong. 

"Han knack' på dörr".... well, now you're from my father's home town of Piteå, and if you don't write the whole song in Pitemål or another dialect from northern Norrland it just looks like a strange form break.

What are the lines connecting to this one? Would you perhaps need a different translation altogether? 

I sort of wished my russian was better, so I could help you out better. It feels to me that your approach is wrong on a fundamental level. It feels like you're trying to translate the words as they stand instead of the meaning the song conveys, if you understand my meaning. Is this perhaps because you don't know Swedish?