Sorry I am Spanish speaker. Nos da is when you go to bed or say goodbye to anybody?
It's when you say goodbye at the end of a working day (for example). I work at the National Library of Wales. We say "Nos da" when we leave for the day (assuming it's late afternoon or the evening).
It's very much like "Hasta la vista" or "Hasta luego" in Spanish. Or in English it'd be "see you"/"see you later".
Edit: It just occurred to me, the most accurate comparison would be "Hasta mañana", in a work setting anyway.
Edit 2: The crazy thing is that "Noswaith da" is used as a greeting, literally as a polite "good evening", that's how evening news begins!
Edit 3: I guess the easiest way to look at is: "Noswaith" is a formal greeting of "Good evening". Whereas "Nos" is used as a general farewell.
I'm probably missing out something very obvious (so sorry if this is a stupid question), but why is there no soft mutation in da?
This is an excellent question since 'nos' is a feminine noun.
There was a very long discussion about this in the facebook group 'dw i'n dysgu cymraeg' some months ago.
The simple reason is that 'nos dda' is actually very difficult to say and thus 'nos da' is used instead.
Remember the point of mutations is to make the language flow better.
However in general use, apart from greetings' the word for night is 'noson'
In this case 'a good night' uses the correct mutation 'noson dda'
One of the most used expressions of 'noson' is the very Welsh event called a 'noson lawen', literally 'a merry evening'
Diolch yn fawr, Rmcode!. So supposedly it would also be "athrowes da" etc.?
Strangely, no! The common cases where this (an adjective starting with d- not mutating after a feminine noun ending in -s) is still usual are:
- nos da - good night
- wythnos diwetha - last week
- penwythnos diwetha - last weekend
And in the wild, some people do now use wythnos ddiwethaf.
nos should be translated as night, and da as good, but that is not what is shown when you hover over it