So there's three possessive a (meaning his, her, their). The way you determine which one is which is from the mutation on the following word (or context depending).
a meaning 'his' lenites the following consonant, and does nothing to a vowel, so you get things like a phasta and a úll.
a meaning 'her' does nothing to the following consonant, but prefixes 'h' to a vowel, so you get a pasta and a húll.
a meaning 'their' eclipses the following consonant, and adds 'n-' to a vowel, leaving you with a bpasta and a n-úll
Hope that helps clear things up!
How on earth do you remember this especially when you don't really understand the grammar in the first place? I thought lenition was something to do with a car.