"He is nasty."
Translation:Mae o'n gas.
can you also say 'mae'n gas' here? like you can say 'mae'n ddiddorol'. ta
Not really, not if you mean 'he' here. The pronoun is sometimes dropped when 'it' is meant, especially if it is something indefinite ("it's too noisy here"), or something generic like the weather ("it's cold today"), say, and it is clear what is being spoken about.