"Building too quickly is not safe."
Translation:Dydy adeiladu yn rhy sydyn ddim yn saff.
rmcode's suggestion is different to what the translation is asking for. In English, rmcode's sentence says, "It is not safe to build too quickly" which carries a different linguistic meaning to "Building too quickly is not safe." To use your sentence, it'd be, "Dyw adeiladu'n rhy gyflym ddim yn saff". The word "diogel" (ddiogel, after yn) could be used too instead of "saff".