"What colour is her hair?"
Translation:Pa liw ydy ei gwallt?
The following pronoun after a possessive is always optional, but the preceding one is always necessary. You might sometimes hear just the following pronoun i.e bag fi instead of fy mag (i). This is wrong or at least considered very poor welsh.
What about "mag fi" with no pronoun in front but the mutation is there? (Or "fag di", etc.)
Probably still wrong. I was always taught in school that you must have the preceding possessive and the following one is optional.
So, is the following pronoun originally an emphatic? Hungarian does something like that: a könyvem = my book, az én könyvem = definitely my book.
There are people who say bag fi, bag di etc., but like EllisVaughan says, it's extremely colloquial. It's sort of the equivalent of saying "ain't" or a double negative in English - first-language speakers do it, but it's unlikely to be accepted by Duolingo or in polite conversation. First-language speakers tend to say fy mag fi over fy mag - the only difference is that leaving the fi off is more formal. But adding it isn't slang and doesn't give any special meaning. Although you could stress the fi if you want to emphasise it. https://en.wikipedia.org/wiki/Welsh_language#Pronoun_doubling
Not sure about mag fi, but mag could occur - the nasal mutation only occurs after yn and fy, so dropping both pronouns is unlikely to cause confusion. I wouldn't expect to see it written down though, and I don't know how common it is. Never heard of it happening for dy fag di or any other persons, probably because the other mutations are common enough that some confusion could arise. https://en.wikibooks.org/wiki/Welsh/Mutations#Nasal_Mutation
Why is "Pa liw yw ei gwallt hi?" wrong? What's the difference between "yw" and "ydy" in this case?
Just a mistake in the database, now fixed.
There is no difference between using either of ydy/yw in this context. One might be more prevalent than the other in a particular area or age group, etc.
The hover hints themselves give as first choice 'Beth yw lliw ei gwallt hi?' Although I feel a bit ashamed of having used the hover, (it was the first sentence in a section I hadn't visited for a very long time...) I can't see anything wrong with what it gave me; but it was then marked wrong. Since it was also given, it should surely be accepted