"This bus is slow, you know."
I didn't useですand it told me I was wrong. My answer was informal, but shouldn't おそいよstill count?
From what I've seen, duolingo wants you to use as much as you can from what's given. Eg sometimes there are sentences with the は particle left out (when it would be grammatically correct to include it), but whenever it's present, you get a wrong answer if you don't use it.
osoi is an adjective (meaning slow/late)- it describes nouns. yukkuri is an adverb (slowly) - it describes verbs/how you do something.
No - they have totally different uses/meanings - ne is like -isn't it? or really? and is a way of seeking reassurance. yo - is for emphasis, like an exclamation mark.
Yo is used incorrectly here. It is more like an exclamation mark and gives added emphasis to a statement - intensifies it/indicates how emphatically you believe what you've said.
Yo can also be used when a speaker is giving new information to a listener, i.e. the "you know" at the end of the sentence.