Adding 으 to predicates stems
If a word's final letter is a consonant this letter is reffered to as a . For example, the word has a 받침 since it ends in the consonant ㅇ while does not have a 받침 since the final letter ㅏ is a vowel. Some suffixes change based on whether a word has a 받침 or not. For example, the honorific suffix 시 will become 으시 when added to a predicate stem that has a 받침.
The predicate stem of 괜찮다 has a 받침 so when combined with the honorific suffix 시, a 으시 will be placed after the predicate stem. Note that becomes in the polite speech level.
?Are (you) ok?
In contrast the the predicate stem for 돌아오다 does not have a 받침, so when combined with the honorific suffix 시 the predicate becomes 돌아오시.
?When does grandfather return?