Translation:You have to be careful when you are climbing.
Your suggestion is perfectly correct, it was the one we left out of the seven possible English translations. That's done now. Thanks for the comment.
I said "rhaid i ti fod yn ofalus pan wyt ti'n dringo" and it was marked incorrect. I'd have thought that "pan wyt ti'n" is more correct than "pan ti'n" which is a contraction used just in informal spoken welsh - or am I mistaken?