The difference is the " 'n " which is a contraction of the word "yn". "Yn" has a couple of meanings, but when it is contracted to " 'n " it can only be one of theses meanings. So "yn"/" 'n " is what's called a verbal particle. They do not exist in English, but basically the "yn/'n" particle is used to connect a form of "Bod" (to be) to a verb, noun or adjective. So "Dw i'n hapus" is "I am happy", "Dw i'n hoffi smwddio" is "I like ironing", "Dw i ddim yn hoffi smwddio." is "I don't like ironing. In the sentence above it is not "Dw i'n" because it is negative so "ddim" is slipped in between the "Dw i" and the "yn" so the yn does not contract to form "Dw i'n".