"The wall is bad."
Translation:Tha am balla dona.
Before labial sounds (P, B, M, F) you use am, otherwise an. It’s just like in- changing to im- in English eg. in imperfect instead of
inperfect, imbalanced instead of inbalanced, immortal and not inmortal – the consonant becomes similar to the next sound, dental n changes to bilabial m.
Just notice that in Scottish Gaelic f is also treated as a labial sound (eg. am fìon the wine, and not
an fìon, vs. English infrequent) because the Gaelic /f/ used to be pronounced with two lips instead of the lower lip and upper teeth (as English /f/).
There are two instances where t- appears, both after the definite article:
- as you mentioned – before vowels (where otherwise no lenition happens), eg. an t-athair the father, an t-ubhal the apple,
- instead of s when it is lenited, eg. an t-sùil the eye, an t-slige the shell, an t-sagairt of the priest, leis an t-sagart with the priest (the t-s- is pronounced as just t).
In both of those instances historically this t- is actually part of the article (which originally was *sindos, *sinda, *sindon, etc. in Proto-Celtic, then lost the endings and initial s- and evolved into ind, int, in in Old Irish, and then in most cases lost the final d/t and became modern an, am, a’.
Coincidentally just today I’ve written another post about history of t replacing s after the article instead of a regular lenition.