What about the phrase "ma na sobie..."
This was how I learned to say someone is wearing something
"is wearing", yes.
"wears", not so much. It's a habitual action, not necessarily performed right now. "Ma na sobie" usually is used to describe what a person is wearing at the present time, "nosi" can be used for both.
Why is duze used instead of duzi? Is skarpety not masculine?
The plurals are 'masculine personal plural' and 'not masculine-personal plural'.
Also no, it's not masculine anyway. "skarpeta" is feminine. But even if it was masculine, it doesn't refer to a man, so it's still the second plural.
What is the difference between skarpet and skarpety?
The grammatical case. "Skarpety" is the Nominative plural, as well as the Accusative - and this is the case used here. "Skarpet" is Genitive plural. See the declension here.
Plural and singular
Why is it one not on?
It sure is "on".
i am wondering in the context of this question if there is any difference or reason to use 'skarpetki' vs 'skarpety'?
Only "skarpety" was taught in this course, so "skarpetki" can just be an accepted option. I'd normally choose "skarpetki", but if there are 'large', then actually "skarpety" do indeed sound better to me.