1) Should be "ddydd Mawrth" (on Tuesday) 2) "Tuesday, I have had tea" is improbable English (at least in GB!). I'm guessing that the deviser of this sentence perhaps meant was "On Tuesday I had tea" (Ddydd Mawrth ges i de).
Whether to mutate an adverb at the start of a sentence or not is a matter of style. Not to mutate it is more old fashioned, apparently. Nowadays the tendency is to mutate it. If an adverb appears elsewhere in the sentence then it should be mutated anyway.
There are many types of mutation in various languages - you will need to look those up somewhere else.
With Welsh, 'mutation' usually refers to the changes in some consonants at the beginnings of words as a result of either:
- The word before it causing some kind of change - this is usually because it makes the sounds flow more easily together (or that it used to in older forms of the language!)
- The purpose that the word serves in the sentence - for example, many adverbs take a soft mutation, as in ddydd Mawrth discussed above, when it means 'on Tuesday' as opposed to simply 'Tuesday'.
Notes on the mutations introduced in this course should be in the notes at the beginning of the various sections of the course. Also have a look in the discussion forums. If you think that anything is missing or unclear, post a message on the discussion forum, perhaps.