"It is very cold."
Translation:Mae hi'n oer iawn.
iawn is generally used as an adjective, and in Welsh adjectives generally (not always) follow the thing they are modifying.
rhy is an adverb, and adverbs tend to come in front of the thing that they are modifying, especially when they are used to intensify that thing. (drud - expensive; rhy ddrud - too expensive.)
However, the real life usage tends to be based on how things have developed over the hundreds of years that Welsh has been a modern language - habit, really! In essence the grammatical 'rules' are often based on that, although sometimes influenced in the past by the grammarians' wishes to tidy things up to align them with classical languages.