"Closing the window tight."
Translation:Cau'r ffenestr yn dynn.
Therein lies my problem. An adverb would be 'tightly' The word in the question is 'tight'; an adjective. Surely it should be left that way?
yn is required either way here.
It is common, in British English at least, to say 'to close something tight' as well as 'tightly'. Also 'pulling something tight'.