"Dechrau" means to begin or start, so shouldn't this also accept, "After beginning to work."? It told me I was wrong.
Although it doesn't sound natural to me, I don't see anything wrong grammatically...
FYI the use of "to" is not needed and you'll be marked correct in a translation of "Ar ôl dechrau gweithio". English translation is: "After starting work. "
Ohh I was wondering this! I thought you might have to say "ar ôl dechrau (y) gwaith"... (y gwaith being your routine job)