I thought "you are" is "dych chi" - can someone explain, why "dach chi" is also correct (I had a "Mark all correct translations"-question here)
Dych and Dach come from the literary froms Rydych chi and Rydach chi. Both are correct with Dych being more south walian and dach being more north walian. I use Rydach/dach.
'dych should be written with the apostrophe to show the missing 'y' at the beginning. but as explained it's the north walian dialect accounting for the 'a' there.
There's no strict rule in Welsh to say that missing letters should be replaced by an apostrophe. And, besides, dych/dach are used so frequently that they can and should be considered words in their own right and alternatives to (r)ydych/(r)ydach.
Why is "Ydych chi yn gwerthu dillad ysgol yma?" a correct answer? The lack of contraction between chi and yn seems odd to me as well as the Y attached to the front of dych.
The "Y-" prefix is common in question forms, though often left out. Not sure about the lack of contraction of "yn".
Probably to show learners that 'n is in fact "yn" and that it's important in Welsh sentences.