In Middle Welsh (dunno about Modern), f is v, ph is f, u is u or v, w is oo, ll is hl, ff is f, dd is like OE thorn, th is like OE eth... You get the picture.

The first letters of words change if they're preceded by words that end in vowels, or if there's a feminine singular noun after the article, or [one of a thousand different reasons]. This means that if you don't know a word, and you look it up in the dictionary, it might not be there because it actually starts with a different letter.