The Same Types of Changes Apply in the Past
The vowel contractions and ㅡ deletion learned with -아요/-어요 also appear when -았어요 or -었어요 combines with vowel-final stems.
First select the past ending and then check whether contraction or deletion applies. For example, 보다 selects -았어요 , forming 보았어요 , which commonly contracts to 봤어요 .
These changes appear in ordinary pronunciation and spelling, not only in fast speech.
Vowel Contractions: ㅗ and ㅣ
For the regular examples here, a ㅗ -final stem combines with -았어요 :
보- + -았어요 → 보았어요 → 봤어요
A ㅣ -final stem combines with -었어요 :
마시- + -었어요 → 마시었어요 → 마셨어요
The full combinations show which stem and past ending produced each contracted form.
The 하다 Pattern in the Past
The stem 하- combines with -였어요 to form 하였어요 , which commonly contracts to 했어요 .
This pattern appears in 하다 -ending verbs and adjectives, such as:
공부하다 → 공부했어요
따뜻하다 → 따뜻했어요
The contracted form 했어요 is the common everyday form.
Dropping the 'ㅡ' Vowel
In the regular pattern taught here, the final ㅡ drops when a ㅡ -final stem combines with -았어요 or -었어요 .
If there is no preceding vowel, -었어요 is the default: 쓰다 → 썼어요 .
If there is a preceding vowel, apply the earlier selection rule. A preceding ㅏ or ㅗ leads to -았어요 , as in 바쁘다 → 바빴어요 , while another vowel leads to -었어요 , as in 예쁘다 → 예뻤어요 .
Tracing Back to the Dictionary Form
These patterns also help you work backward from a past form. First restore the uncontracted combination, then identify the stem and add -다 to check the dictionary form.
봤어요 → 보았어요 → 보- → 보다