It may be the system only accepting certain kinds of answers, or that to be in a continuous state it should be 'stanno ...' and then a verb in continuous tense. There is no continuous tense in this sentence. It is present tense. I hope this makes sense, one way or the other :)
"bene" is an adverb, adverbs never declinate according to gender/number. But they can be modified in other ways like here... For example "bene" can change as follows
Benissimo = very well
Benino = more or less ok
Benone = above any expectations
Anyway, all those variations are pretty idiomatic, apart "benissimo", and I wouldn't use them too often before knowing the language very well.
Thank you! For some reason I thought adverbs are declinated as well, but this makes a lot of sense. :) Also, I hadn't heard of "benino" or "benone" before (maybe because, as you say, they are not as frequent) - I only knew you could do that to nowns as in "un bacio": "un bacione" and "un bacino". Allora grazie mille!
my first answer was "Thank you, they fit very well". I imagined a situation in a shop when a clerk asks the customer: "How do you find the gloves?", and the customer answers "Grazie, stanno benissimo - Thanks, they fit very well!". I guess that would be acceptable in reality?
I am a native (American) English speaker but I read a lot of British English literature.
I have NEVER seen "Thankyou" as one word. I checked the Cambridge English dictionary and it agrees it is ALWAYS written as two words: "Thank you." unless you are colloquially shortening it to just "Thanks."
Where is your reference that said it is one word??