The Korean Writing SystemSyllablesNounsPredicatesFormal and Informal SpeechPre-Final and Final EndingsSpeech LevelAdding ㅓ or ㅏ to a predicate stemAdding 으 to predicates stemsThe Deferential Speech LevelThe Intimate Speech LevelThe Polite Speech LevelThe Plain Speech LevelIrregular PredicatesThe Subject Particle 가 and 이The object particle 를 and 을The Topic Particle 는 and 은Forming a Sentence이다 and 아니다있다 and 없다NegationPast TensePredicates as ModifiersDeterminersBound NounsDesireNumbersCountersQuestionsThe Particle 에

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?