"i" means "I" or "me". "i'n" is an abbreviation of "i yn" seen in sentences like "dw i yn" except you'd never use "dw i yn". Same with other pronouns. So Ti'n=Ti yn (and also buttock with out the apostrophe) Chi'n=Chi yn Ni'n=Ni yn Nhw'n=Nhw yn Hi'n=Hi yn Fo'n/o'n=Fo yn/o yn Fe'n/e'n=Fe yn/e yn

the same like "eisiau" or "isio" - just in another Welsh dialect (I don't know which one).

C mutates to a G because Ysgol is feminine, iirc

It's a mutation, the C changes into G