How do you say ‘it’ in the sense of “I have finished it” or “Have you done it yet?”
You are unable to say ‘it’ in Welsh. In your examples the ‘it’ refers to some noun and that noun will have a gender, either masculine or feminine. All nouns in Welsh are either masculine or feminine (or even, rarely, are both). Therefore, in the North, you use either ‘o’ (or ‘fo’ after a vowel) for ‘it’ where the noun is masculine or ‘hi’ where the noun is feminine. Thus to speak absolutely correctly you need to know the gender of all nouns you use .
However which learner remembers the gender of all the nouns they use? No problem, just use the one that comes to mind and the result is fully understandable. I go for ‘o’ when I do not know the gender (or think fast enough), on the basis there are more masculine nouns (I think) than feminine nouns and therefore get more right than wrong. Hope this helps.
Depending on context, you can also use ‘y peth’ or ‘the thing’.
Wy ti wedi gorffen y peth hyd yn hyn?
Have you finished it (the thing) yet?
This is a good suggestion, and is something also used by first language speakers, so there’s no need to be nervous about it sounding odd or ‘learnerish’.