why på instead of om?
om would mean "about" here.
You could say om tisdagarna, though - it's a slightly oldfashioned way of saying the same thing. I do not recommend using it.
Is Pa Tisdagar sjunger han? unacceptable?
It could work in spoken language perhaps, but the norm is that you always begin with the verb to form a question.
Should "sjunger han på tisdagarna?" be accepted?
No, it slightly changes the tone and meaning, the same way it does in English. På tisdagar/on Tuesdays, just generally - på tisdagarna/on the Tuesday, specifically.
Can it be: "Gör sjunger han på tisdagar?"
No, we don't have a 'do' construction.
Ohhh, okay! Thank you Arnauti, you beautiful person you!
Why isnt tje transelation " he sings on tuesdays" ?
Where does the "does" comes from ?
It's a question - English commonly uses variations of "do" to create them.
Why can't I say Sjunger han om tisdagar
That means "Does he sing about Tuesdays?"
Are we allowed to use the straight word order plus väl construction to form questions, or only if the English question contains something along the lines of "doesn't (s)he / isn't it /" etc.?
I put: "Han sjunger väl på tisdagar?"
Only if it has an English equivalent in the source phrase, like you suggested.