I really don't know, I assume it's just historical spelling, but that's just a guess on my part.
This is the reason i got the spelling wrong... I hear an fh not a f. Suppose they are all like that so shouldn't have been surprised.
fh in Irish is completely silent - in the case of féin, the pronunciation* is actually héin, so you aren't actually hearing an fh.
* Note that féin is pronounced with an "f" sound in some cases, depending on the dialect, and the preceding word - féin is even pronounced with a "p" sound in some dialects in certain specific circumstances.
why is there a D'? Why do some sentences have the "d" and others not?
Past tense verbs are lenited if they start with a consonant that can take a séimhiú. If the verb starts with a vowel, the past tense is indicated with a "d'".
Historically, "do" was used to indicate the past tense for all verbs - that's the source of the lenition, but nowadays the "d'" before a vowel is all that remains of the "do" (except in some dialect forms).