At about 27:10 in Level 1, Challenge 15, there is the sentence “And he said that he wants to learn Welsh.”
I would have said “A ddudodd o fod o isio dysgu Cymraeg,” but imagine my surprise when it was given as “Ac mi ddudodd o fod o isio dysgu Cymraeg.” It was said that way both times.
Where did the “mi” come from? Did I miss something in the previous challenges or misinterpret the English? It was definitely not mae (and I’m not going into the “Why is it ac before an M?” again!)
The “mi” is an optional addition often heard in speech in the Northern version of Welsh. The equivalent in South Wales Welsh is “fe”, which is not the same “fe” as the word for “he”. “Mi” causes a soft mutation.
To confuse things further, very often people will omit the “mi” and just sort of imagine it’s there and still use the soft mutation for the verb. The thing is, don’t worry about it and just copy what you hear in the challenges. “mi ddudodd o” and “ddudodd o” are both correct in speech. You’d need to be more precise in writing, but worry about that later!
There may be a “rule” about why it’s ac before an “m”, but I just stick with, “It’s just how people say it.” 
The SSiW courses try to expose you to all the different forms you may come across, and the use of ‘mi’ is one of them - I’ll explain, but firstly, don’t worry about it, your answer was just as correct 
Mi (and ‘fe’ in the South - but not to be confused with the fe meaning ‘him’), do not translate to English nor actually mean anything either, they are just kind of ‘filler’ words that often appear before short form verbs in a positive statement (and they cause a soft mutation).
You may have heard the nursery rhyme “Mi welais Jac-y-Do” - same thing applies there - “I saw a Jackdaw”, no translation of ‘mi’.
Ah - I’ve just seen Margaret was quicker typing than me, so I’ll leave it there!
Diolch, @siaronjames a @margarethall (or should it be ac @margarethall? 
)
These little bits and pieces that seem random but apparently aren’t always throw me. Thanks for being around to answer my questions. It’s helpful to know it doesn’t mean “me” (which I wondered, given the context), and also to see the North vs South variants.
Incidentally, the “Ac before mae” query turned into a huge, long, and very interesting discussion. You can find it here if you’re so inclined to see the tangents it went off on.