This dictionary doesn't aim to cover every single Welsh word and their conjugations and mutations, but it concentrates on those words which appear in the BBC Catchphrase series of lessons. If you search for a word (try "bachgen" or "house"), it will tell you the Welsh and English equivalents and give you the links to the lesson(s) in which that word appears. If there's an entry in Wiktionary, it will give you the link to that too.
Go on, search for something!