The system should always accept both 'prynhawn Xxxx' and 'prynhawn dydd Xxxx' as answers to any '... afternoon'. There is no difference in meaning between them and both forms are commonly used in the wild. Similarly with bore Xxxx and bore dydd Xxxx for '... morning'.
If you do come across an example where one of those forms is not accepted, just use the 'report' button so that we can add the missing form to the database.
The same as in English, more or less.
With days, Welsh has two separate words instead of one, so just the name element is capitalised unless it is the first word in a sentence or if it is a 'special' day of some sort:
- dydd Llun - Monday
- nos Sul - Sunday night
- Dw i'n mwynhau Nos Galan - I enjoy New Year's Eve
- Dydd Gwener ydy fy ffefrin. - Friday is my favourite.
(There are a couple of mistakes with this in the course which pop up sometimes, but they will be fixed in the future.)