One Rule for Two Tenses
The rule you learned for the present tense ( -아요/-어요 ) is exactly the same rule you use for the past tense ( -았어요/-었어요 ).
You do not need to learn a whole new conjugation system. If you know how to choose 아 or 어 for the present, you just add the past tense marker ( ㅆ ) to that same choice.
Core takeaway: Present and past use the same a/eo choice
Basic Consonant Stems
For stems ending in a consonant, you simply add -아요 or -어요 for the present, and -았어요 or -었어요 for the past.
Vowel ㅏ or ㅗ
잡다 → 잡아요 → 잡았어요
Other Vowels
먹다 → 먹어요 → 먹었어요
Vowel Contractions and Drops
The exact same vowel contractions and drops you learned in the previous lessons happen in the past tense too. Because the past tense starts with the same 아/어 sound, the vowels merge in the exact same way.
Contractions
Dictionary · Present · Past
보다 · 봐요 · 봤어요
마시다 · 마셔요 · 마셨어요
Dropping the ㅡ Vowel
Dictionary · Present · Past
쓰다 · 써요 · 썼어요
The '하다' Verbs
Just as 하다 changes to 해요 in the present tense, it changes to 했어요 in the past tense. This applies to all verbs and adjectives that end in -하다 .
Dictionary · Present · Past
하다 · 해요 · 했어요
운동하다 · 운동해요 · 운동했어요
Note: A common mistake is trying to add -았어요 to the original stem (하았어요). Remember that 하다 transforms to 해- before adding the final ending!