Hi, i was studying grammar and suddenly, i felt a little strange with some sentences which are "hastaymış", "fansadaymış" and "trendeydim"

but i thought y buffer is placed between vowels.

isn´t it about the fusion consonant? or what did i miss here? 

You are correct, the y is a buffer letter.

Though in this case the suffix does not begin with a vowel though, it is used when adding to something ending in a vowel.

I´m confused myself how we would know the meaning here. As miş can be used for: surprise, reported information/uncertainty and using it similar to "had". Not sure how to tell here.

You are correct that the buffer letter ´y´ is used to separate vowels.

In these cases it stems from the past use of the old Turkish verb ´imek´ (to be), which starts with a vowel.

Click here for an explanation

Thus hasta (sick) + imiş (he/she apparently was)

= hasta + y + mış (note the first ´i´ has now been dropped from imiş

= hastaymış

1- ((Hastaymış) --> Noun(Vowel letter) + Y + imiş

if noun ending in vowel letter we add to noun buffer letter (Y) then add past tense suffix.

2 - ((Trendeydim)) --> Noun + de + Y + idi