Korean Past Tense of Verbs/Adjectives -았/었어요

The past tense form of adjectives and verbs is formed by adding -았/었 to their word stems.

When the word stem ends in the vowel ㅏ or ㅗ，-았어요 is added, and when it ends in any other vowel,-었어요 is added. For verbs and adjectives ending in 하다, 하다 is turned into 했어요 (Originally, -였어요 is added to form 하+였어요, but it is shortened to 했어요,) , e.g.:

앉다 + 았어요 -> 앉았어요 (sat)

먹다 + 았어요 -> 먹았어요 (ate) 

공부하다 + 았어요 -> 공부했어요 (studied)

In the case of the formal polite style, -았/었습니다 and -했습니다 are used, e.g.:

앉다 + 았습니다 -> 앉았습니다 (sat)

먹다 + 었습니다 -> 먹었습니다 (ate) 

공부하다 + 했습니다 -> 공부했습니다 (studied)

For 이다, if the noun ends in a vowel, -였어요 is used and if the noun ends in a consonant, -이었어요 is used. E.g.:

간호사였어요

학생이었어요

For 아니다, if the noun ends in a vowel, -가 아니었어요 is used and if the noun ends in a consonant, -이 아니었어요 is used. E.g.:

간호사가 아니었어요

학생이 아니었어요

While the past tense of 주다 can be written either as 주었어요 or 줬어요) and the past tense of 보다 can be written either as 보았어요 or 봤어요; the past tense of 오다 is written only as 왔어요， not오았어요.

• 주다 + -었어요 -> 주었어요 or 줬어요

• 므다 +-았어요 -> 보았어요 or 봤어요

• 으다 았어요 -> 왔어요, 오았어요(x)