Expresses that something is pleasing, impressive, or of high quality. Often used in polite or slightly formal contexts.
This is a splendid garden, isn't it?
Thank you for the lovely gift.
Adverbial use meaning 'to a considerable degree' or 'more than expected'. Often softens a statement.
This movie was quite interesting.
He arrived fairly early.
structure; construction; framework
Noun meaning the way something is built or organized. Less common than the adjectival/adverbial uses.
The structure of this building is very sturdy.
I'm fine without more tea, thank you.
It's fine to just leave it as it is.
I received a lovely souvenir.
This problem is quite difficult.
けっこう is a casual, everyday way to say 'quite/fairly', while なかなか carries a slightly more formal or admiring tone, especially in なかなかの〜 constructions.
結構 can mean 'quite/fairly' like 割と, but it also has separate meanings like 'fine' or 'enough' that 割と does not share.