There are few typos in the notes to this lesson:
Dyn ni wedi mwynhau cinio. - We have enjoyed lunch. (But not 'We swam...') <--- HERE - Doesn't make sense in relation to sentence.
ym- is quite a common verb prefix in Welsh. It often indicates a reflexive action action - something that someone does to themselves. This is not HERE - Always ---> slways obvious in the English translation.
In golchi (washing) and ymolchi (washing HERE - Oneself --> ineself, getting washed) it is very clear, though: