"Dych chi'n mynd ar wyliau cyn dechrau gweithio?"
Translation:Are you going on holiday before starting to work?
'...before you start work' / '...before we retire' : marked incorrect. - to make sure we know the difference between 'cyn dechrau gwaith' and 'cyn i chi dechrau gwaith' / 'cyn ymddeol' and 'cyn i ni ymddeol'? (though I thought duo was asking me to write the english for 'cyn i ni ymddeol'). Diollch.
As there is no change of subject in the two examples it would be perfectly natural English to translate as you have. I believe they are telling us that using the longer form is not considered good Welsh. I don't know what happens if you pretend to be a Welsh speaker trying to learn English by adding a new course.
A native English speaker would not translate 'dechrau gweithio' as 'starting to work' in this context. Simply 'starting work' is a correct Welsh to English translation and should be accepted.
That is in several of the accepted answers here, but the Duo systyem sometimes refuses to accept the alternative answers. Please raise a 'Duolingo bug report' if it happens again, so that Duo staff can investigate the bug. It is not something that the course teams can fix.