The idea of mutations can be somewhat daunting to some, but Celinese's system of changing the initial consonant of a word in certain circumstances is both predictable and relatively simple. All you need to know is what triggers mutation and what mutated letters change to.
What brings about mutation?
Nine times out of ten, initial mutations are triggered by prepositions. Elithoan teachers abroad often jocularly refer to prepositions as stubborn - because they cannot precede many nouns, pronouns and adjectives that follow them unless the latter change their initial letter. Cé (with), lo (to), go (of, from), na (in), mareir (except), och (out [of]), dysar (up, over), bair (around, about, near) are just a few in this number - the easier way to remember is that almost always, a preposition will lead to a mutation.
You can entirely count on all possessive pronouns (like mo, to/ano, so, etc)triggering mutation too, and optionally, if an adjective precedes a noun, as a matter of style there can be mutation if you choose. Another optional mutation occurs when a number higher than two precedes a noun or adjective, but this is confined to the formal acrolect.
Examples of mutation
Na b-poreg (g-)cafel -in the small room (optional double mutation!) Go d-tainoc lo ð-thonoir byroí - I walked from the forest to the restaurant. Cêith sío mo g-caroig - my heart is with you.
In all these examples we see a voiceless initial consonant being replaced by its voiced counterpart - p, c ([k]), t and th turning into b, g, d and ð. One exception is initial f (pronounced [f]), which mutates to w - this is, however, pronounced [v] when the mutated f precedes an r - as in cé w-frensos [ke ˈvɾɛnsɔs] (with the break). When mutated f precedes l, it is not pronounced but the subsequent l is pronounced as a velar: mo w-flarë, my hand, is [mɔ ˈʟaɾə].
In standard Celinese writing, it is only these five consonants that change written form. In reality, /h/ also changes to /ɦ/, but this is not reflected in the orthography.
An important thing to note is that the consonant before the dash replaces the consonant afterwards' sound, so 'na g-celín' - in the language - is pronounced as 'na gelín.' This was a compromise made when Celinese spelling was codified between those who wanted Celinese to have as few redundant letters as possible (who would have preferred 'na boreg gafel' and those who wanted to show words not necessarily as they are pronounced, but as what they derive from (who would have preferred 'na poreg cafel.')
Not a mutation, but relevant
You may well have come across similar phrases to the above where r- (Southern Celinese and Pairseg) or th- (some Northern Celinese varieties) follows a preposition ending in a vowel and latches onto a noun or adjective starting with a vowel. This is not mutation, but epenthesis. This is an epenthetic consonant, almost always inserted in these circumstances to split the two vowels, such as in my garden, "mo r-aloir". ("Mo aloir", however, is not incorrect, and is sometimes seen in poëtry, pronounced as [ˈmwalɔiʐ], as a way of fitting one more syllable into a line.
Another circumstance where this epenthetic consonant is used is when an object pronoun ending in a vowel (such as mé, me, or ané/té, you) comes before a verb beginning with a noun (for example, ethlí, name, or acluthí, listen). Whilst it is not obligatory, it is considered more proper to either insert the epenthetic consonant between pronoun and verb (as in mé r-ethlín, I'm called, or té r-acluthín), or, preferrably still, to use a pronoun suffix instead, i.e. ethlínom or acluthínoth.