As I understand it, lenition is something which has been “evolving” to smooth the letters of the language for many centuries. What we know of lenition today is perhaps the remaining part. Simply the insertion of the letter H into the word after the first letter. Much older forms of lenition have been absorbed into vocabulary and you can see evidence of these in words like sèimh or fliuch which looked very different in the past and whose endings were lenited.

Remember the list of consonants which can be lenited from last time, not all consonants are lenited. C, B, M, D, G, S, T, P, F The letters L, N and R also lenite but are not written, you need to keep this in mind.

Below are the conditions of lenition

Choisich mi past tense *we will discuss that later, along with other tenses which can be lenited

oidhche mhath adjectives with feminine noun

mo bhràthair  possessives mo, do and a, *only these possessives,  always lenite. Possessives also cause the word to insert a vowel near the end of the word, Its called slenderisation. We will talk about that later.

bho, de, do, fo, gun, mar, mu, ro, tro prepositions which cause lenition.

corra deagh, droch, priomh, sàr, seann  adjectives which lenite

glè mhath modifiers glè, fior, ro

aon, dà, a’chiad  these numbers lenite

bu  this verb lenites

cha negative particle lenites.