"Ym mis Mawrth"
This is explained in the notes somewhere.
In a couple of cases, yn - but only when it is used as a preposition meaning 'in/at' - changes its form, depending on the letter at the start of the following word:
- yn + m- becomes ym (ym Mangor, ym Mhont-y-pŵl, ym Mhenfro, ym mhobman, ym mis Ebrill, ...)
- yn + ng- becomes yng (yng Nghaerdydd, yng nghanol y dref, yng Ngerddi Soffia, ...)
Why am I getting an X for 'in the month of March' when Ym mis Mawrth has "in [the] month [of] March" in its help bubble (and more verbosely translates to it)? This makes no sense to me.