"I have washed the floor."
Translation:Dw i wedi golchi'r llawr.
ym- is a reflexive prefix, so ymolchi is 'to wash oneself':
- Dw i'n ymolchi - I'm having a wash/washing myself
- Mae Dewi yn golchi'r car - Dewi is washing the car
Is there a rule for using "dw i" vs "dw i'n"? The system didn't like "dw i'n" for this one.
wedi goes in the same "place" as the yn that's abbreviated to 'n after a vowel. So you can't have both.
A bit like how in English you have "I am eating" and "I have eaten" but not "I am have eaten" -- if you use "have" for perfect tense, you don't use "am" for present tense.
wedi is used for perfect tenses in Welsh (present perfect + past perfect), yn for most other tenses formed with bod "to be" such as the present tense, imperfect, or future, to link bod with the verb.
Choose only one :)
(And then there are eisiau and angen which do not use yn, for historical reasons.)
There is a good explanation of when to use Dw I versus Dw i’n at this YouTube of WelshPlus: https://m.youtube.com/watch?v=AZmfScSW2wc. Briefly, Dw i’n for actions, persons/things, describing a word or number. Dw i for places, mor, wedi and prior to eisiau, and angen. Hope this helps. It baffled me for a while. . ..