Where can I study and practice Welsh mutations on Internet?
Does anyone know where to practice it? It's very interesting but difficult to learn.
I wouldn't say you have to practice it specifically, you'll get used to it with time - normally a word will just "feel right" if it's meant to be mutated (my Welsh teachers always told us that mutations were there to make words easier to say afterall!). Either way, there's just a few simple rules, you'll find a bunch of tables online, top of this page has such a table, then you just need to learn the rules for when each mutation takes place.
I believe they originally were intended to make words easier to say, but then the bits that caused the mutations dropped off while leaving the mutations behind :)
Most obvious case in point: "ei" (his/her) causes either lenition ("his") or aspiration ("her") - the "ease of pronunciation" explanation would only explain one mutation per word.
I think that lenition, for example, came about from when a consonant was between two vowels (one of them at the end of the previous word). Thus the mutation of adjectives after feminine nouns, for example, if those used to end in a vowel similar to, say, the first declension in Latin (mostly feminine nouns) that ended in -a as in "villa".