Irregular verbs '으'

(1) When verbs ending with '으' meet the vowel, '으' is omitted. Most verbs ending in a final vowel '으' of the stem are irregular.

쓰(다) + -어요 : ㅆ+ㅓ요 => 써요 : to write,to use,to be bitter,to put on(a hat)
크(다) + -어요 : ㅋ + ㅓ요 => 커요 : to be tall
뜨(다) : to rise
끄(다) : to turn off
저는 편지를 써요.
I am writing a letter .
편지를 썼어요.
I wrote a letter.
편지를 써야 해요.
I have to write a letter.
동생은 키가 커요.
My brother is tall
Learn more about Korean alphabet here: Korean Alphabet
(2) '-아요' is used if the vowel before '으' is 'ㅏ' or 'ㅗ', '-어요' is used if the vowel before '으' is other vowel except '아' and '오'.
-으 irregular verbs + '-아요'
바쁘(다) + -아요
: 바ㅃ + ㅏ요 => 바빠요 : To be busy
배가 고프(다)
: to be hungry
나쁘(다)
: to be bad
잠그(다)
: to lock
아프(다)
: to be painful
저는 오늘 바빠요.
Today I am busy.
오늘 아침에 바빴어요.
I was busy this morning.
바빠서 못 갔어요.
Because I was busy, I couldn't go.
-으 irregular verbs + '-어요'
예쁘(다) + -어요 : 예ㅃ ㅓ요 => 예뻐요 (To be pretty)
슬프(다) : 슬ㅍ ㅓ요 => 슬프다 (to be sad)
기쁘(다) : to be happy
슬프(다) : to be sad
Learn how to read in Korean here.