Translation:Ym mis Mawrth
Mewn is unspecific (think "In a") whereas Yn is specific (think "In the") so since Mawrth is a specific time of year you use "yn"
Thank you for the clarification but what's the difference between Yn and Ym?
Yn is more or less considered the base word. It changes a little depending on the word that come after it. So "Yn yr ysbyty" is "in the hospital" and this is the form of (specific) "in" you will see most.
"Yn" becomes "Ym" before a word that starts with "M" so "Ym Maldwyn". It also takes this form if the word following it mutates to start with an "M" so "In a bottle" is "Yn + potel" which makes "Ym mhotel".
There is also another form of Yn which is Yng. This (as you can guess) comes before words that start with ng(except none of these exist to my knowledge) or mutate to start with yng. So "In a castle" is "Yn + Castell" which is "Yng nghastell".
Here is a link to a mutations grid to show you what all of the nasal mutations are: https://31.media.tumblr.com/tumblr_m9ox8wy0rR1rw84ef.png
Also a way to remember the letters that mutate are T, C, P Doctors Go Bananas, Rhian Marries Llywelyn.
Thanks for such a good explanation. I swear mutations will be the death of me.
Most of the population of Wales agrees with you, but one day they'll just click I'm sure
It stops being difficult if you consider that "p", "b" and "m" are all pronounced with your lips, and "c", "g" and "ng" are all said in the same region towards the back of your mouth.
All languages (not just Welsh) tend to shift their sounds so they're more similar to adjacent sounds. Like how in English, "in caves" is really pronounced "ing caves" - although nobody bothers to write "in" that way! In fast speech, people often say "im places" too.
Welsh is only unusual in representing this shifting of sounds in writing, rather than leaving it unwritten.