"the small tooth"
Translation:an fhiacaill bheag
The feminine singular definite article causes lenition. This normally involves adding an h to constanants other than l, n and r.
Unfortunately the article is not normal and the rule for d, t and s will be covered later.
When you add the h you change the an to a', except for fh which is silent.
I apologize deeply that it is so complicated but it is not my fault.