1.
There is a letter change (soft mutation) after 'n (or 'yn') in sentences like:
3 / tri = Mae hi'n dri o'r gloch

2.
There is a letter change (soft mutation) after 'i' in sentences like:
3 / tri = Mae hi'n chwarter i dri

3.
There is a letter change (soft mutation) after 'am' in phrases like:
3 / tri = ... am dri o'r gloch

4.
Here is a list of all the times. The more traditional ways of saying numbers, e.g. ugain instead of dau ddeg are used. There are slight changes, e.g. ugain (20) becomes 'hugain' here; numbers often lose the last consonant, if there is a consonant following, e.g. pump - pum munud.

Pum munud wedi - Five past
Deg munud wedi - Ten past
Chwarter wedi - Quarter past
Ugain munud wedi - Twenty past
Pum munud ar hugain wedi - Twenty five past
Hanner awr wedi - Half past
Pum munud ar hugain i - Twenty five to
Ugain munud i - Twenty to
Chwarter i - Quarter to
Deg munud i - Ten to
Pum munud i - Five to

5.
If you're asking what time it is now, the question is "faint o'r gloch ydy hi", but if you want to ask what time something is happening, the question is "faint o'r gloch mae..." faint o'r gloch mae Eastenders, faint o'r gloch mae'r parti, faint o'r gloch mae'r dosbarth yn dechrau.