Haitian Creole ↔ English Reference, Look up Haitian Creole and English Words
Answer to the first question:
No. Using “yon” or not will have to depend on whether you’re talking about non-specific nouns and how many of these “non-specific” nouns you’re referring to or are you using a general term for them.
You can actually say “M pa’t ka menm achte yon grenn jilèt aswè a“. – I could not even buy one single blade tonight.
M pa gen yon dola sou mwen. – I don’t have a dollar on me.
Pa gen yon kretyen vivan nan lari a aswè a. – There’s not a single soul in the street tonight.
M konnen se yon papiyon ou wè lè w gade imaj la. Mwen menm m pa wè yon papiyon, se yon fèy mwen wè. – I know you see a butterfly when you look at the image. As for me I don’t see a butterfly, I see a leaf.
So you ARE able to use “yon” in negative sentences.
And you will not use “yon” if you’re using a non-specific noun in general term or if it’s plural, etc....
M pa bezwen jilèt. - I don't need blades.
M pa wè moun deyò a. - I don't see anyone outside
Nou gen kola pou tout moun. - We have sodas for everyone
Answer to the second question.
You are right. You should say “M pa kapab” instead.
"kapab" will go at the end of the sentence instead of "ka".