"Her yellow car was very old."
Translation:Onun sarı arabası çok eskiydi.
-sı in arabası is a possessive ending, please read tips and notes of that skill and consider reviewing it :)
-y in eski-y-di is a buffer letter, you always need it before a -di suffix if the word ends with a vowel
if you say eskidi, then you use "eski" as a verb (eskimek), so it means "her yellow car got old"
Is it correct in thinking that when ever the stem of the verb ends in 'I' a 'y' is added before the past suffix? Or is this just in some cases, irregular case perhaps?
Not when the predicate is a verb; only when the predicate is an adjective or a noun.
For example, boyamak "to paint, dye" has boyadım "I painted, I dyed" -- that's a real verb.
But for pahalı "expensive", for example, the past would be pahalıydı "it was expensive" -- an adjective.
And for anne "mother", you would have anneydi "she was a mother" -- a noun.
I believe this used to be two words, as in pahalı idi, anne idi, from a pseudo-verb imek.