요 form in korean

If the last syllable of the stem contains the vowels  ㅏor ㅗ, then 아 is added.

Verb/Adj Stem + 아
괜찮다 (to be fine, ok)→괜찮아(요)
맞다 (to be right, correct)→맞아(요)
괜찮다 (to be fine, ok)→괜찮아(요)
놀다 (to play)→놀아(요)

If it is a stem with no 받침like 가다, then an extra 아 isn't added
가다 (to go)→가(요)[ka da→ka yo]
사다 (to buy)→사(요) [sa da→sa yo]

Stems that end in 오 are often contracted to 와 (오 + 아 = 와)
보다 (to see)→봐(요) / 보아(요) [boda→bwa]
오다 (to come) →와(요) / 오아(요)[oda→wa]

*Please note this rule does not apply for 하다 verbs


Verb/Adj Stem + 어
If the last syllable of the stem contains any vowel other than 오, 아, and is not a 하다 verb then it becomes an 어
먹다 (to eat)→먹어(요) [meog da→meog eo yo]
울다 (to cry)→울어(요)
필요없다 (to need)→필요없어(요)
힘들다 (to be difficult, tiring)→힘들어(요)
주다 (to give)→줘요 / 주어(요)[jo da →jwo yo]
쓰다 (to write)→써(요) [sseu da →sseo yo]
크다 (to be big)→커(요)
신다 (to put on shoes/socks)→신어(요)
읽다 (to read)→읽어(요)
재미있다 (to be fun, interesting)→재미있어(요)


If the last syllable of the stem contains the vowel 이 and there's no 받침, i.e. 시키다, then it becomes a 여 (시켜). The 여 is a result of the contraction between 이 + 어.
내리다 (to descend)→내려(요) [nae ri da → nae ryeo yo ]
놀리다 (to tease, make fun of)→놀려(요)
떨어지다(to fall)→떨어져(요)

Verbs that end in in the vowel ㅐ don't get anything conjugated to it, i.e. 내다 → 내요.

여 Verbs/Adjectives (하다)
하다→해(요)

refer from
http://www.koreanwikiproject.com/wiki/index.php?title=%EC%95%84/%EC%96%B4/%EC%97%AC_%2B_%EC%9A%94