"Dw i'n hoffi dydd Llun."
Translation:I like Mondays.
Yes, "dw i ddim yn hoffi dydd Llun" can be "I don't like Mondays". If you think about it, the two English sentences:
- I don't like Monday
- I don't like Mondays
are exactly the same, it's just in English we tend to pluralise the day in order to make a generalisation, but Welsh doesn't.
Not usually, as except for Sul (an old word for 'Sun') and Llun (and old word for 'Moon'), the names of the days are also names of planets. Also, llun means 'a picture' and iau is one of two common words for 'a liver'.
You will come across the names of days and months being used without dydd and mis where the context is absolutely clear but it is generally better to use them. Abbreviating the names of days to, say, three letters doesn't seem very common - maybe have a look at a bus or railway timetable and check!