"In the evening"
Wondering what the gog version of this is? You won't hear gyda for sure. Do people say 'efo'r nos' ?
King says in his grammar (§454, s.v. efo):
Efo (in some areas hefo) is the general word for with in the N. In the S it is always replaced by gyda, a word which, in turn, is virtually unknown in the N (except in the set phrase gyda’r nos at night).
Note the last bit in parentheses :)
All true, except I imagine that you would struggle to find people in the north who don't at least know gyda.
if the hints accept that evening means noswaith then gyda'r noswaith should be accepted, unless you have a clear statement in lesson notes that noswaith is only acceptable in certain defined situations, like "Good evening Class" from teacher in Welsh lessons!
No, see earlier comments. gyda'r nos is an idiomatic expression meaning 'evening' and 'in the evening'. Another example of where a literal translation does not work.
The hints are not context-sensitive, so just because they say "noswaith = evening" does not mean that that is true in all situations.