In short

The Korean past tense is one syllable slipped into a word you can already build: take the 아/어 form from lesson 10, add ㅆ, and finish with 어요. 먹어 -> 먹었어요, 가 -> 갔어요, 해 -> 했어요. There is no separate perfect tense - this one form covers ate, have eaten and had eaten.

The rule

Step 1 - The past marker sits INSIDE the word, before the final ending

Korean is agglutinative: a predicate is a stem with slots stacked on it, and the sentence ending always comes last. Tense goes in the slot just before it.

Stem + [아/어 form] + -ㅆ- + -어요

So 먹었어요 is not one lump but 먹 + 었 + 어요: stem, past, politeness. That is why you will later see 먹었는데, 먹었지만, 먹었습니다 - the same 었 with a different ending bolted on.

Step 2 - Choosing 았 or 었 is the lesson-10 rule again, nothing new

-았- and -었- are the same 아/어 pair you already know, with ㅆ riding on top:

1. Stem’s last vowel is ㅏ or ㅗ -> -았-. 받다 -> 받 + 았 -> 받았어요 (badasseoyo, received).
2. Anything else -> -었-. 먹다 -> 먹 + 었 -> 먹었어요 (meogeosseoyo, ate).
3. 하다 -> 했-. 하 + 였 -> 했어요 (haesseoyo, did), and so every noun + 하다 verb: 공부했어요 (gongbuhaesseoyo, studied).

Only the last vowel of the stem counts, exactly as before.

Step 3 - The contractions are the same ones, and still obligatory

If the stem ends in a vowel, the 아/어 merges into it first, and then ㅆ lands on the merged block. *가았어요 is simply impossible, and although the uncontracted 마시었어요 is tolerated on paper, nobody writes it - use 마셨어요. The full list is in Forms & patterns - it is the lesson-10 contraction table with ㅆ어요 added.

Step 4 - The shortcut: 해요체 minus 요, plus ㅆ어요

Because the past is built on the same 아/어 form as 해요체, you can go straight from the polite present to the polite past:

[해요체 form] - 요 + ㅆ어요

가요 -> 갔어요, 마셔요 -> 마셨어요, 배워요 -> 배웠어요, 돼요 -> 됐어요, 해요 -> 했어요. This works for every verb and every adjective. The copula is the one exception (Step 5) - do not run 이에요 through it.

Note that the ending after the past marker is always 어요, never 아요. The 아/어 choice is made once, before the ㅆ; after it, 었어요 is fixed. *갔아요 is always wrong.

Step 5 - 이다, 아니다, 있다, 없다

The copula’s 해요체 이에요/예요 is not built on an 아/어 form, so the Step-4 shortcut does not apply; its past is chosen instead by the 받침 of the noun in front:

Noun with 받침 + 이었어요 / Noun ending in a vowel + 였어요

학생 (받침 ㅇ) -> 학생이었어요 (haksaengieosseoyo, was a student); 의사 (vowel) -> 의사였어요 (uisayeosseoyo, was a doctor). 였어요 is simply 이었어요 contracted after a vowel. The negative is 아니었어요 (anieosseoyo), still with 이/가: 학생이 아니었어요 (haksaengi anieosseoyo).

있다/없다, by contrast, are perfectly regular: 있었어요 (isseosseoyo, there was, had) and 없었어요 (eopseosseoyo, there wasn’t, didn’t have).

Step 6 - Adjectives take the past too

Korean 형용사 are verbs (lesson 11), so they inflect for tense just like action verbs - where English needs was + adjective, Korean needs no 이다 at all: 추웠어요 (chuwosseoyo, it was cold), 좋았어요 (joasseoyo, it was good), 바빴어요 (bappasseoyo, I was busy).

추웠어요 comes from 춥다, a ㅂ-irregular (춥 -> 추워); its regular look-alike 좁다 gives 좁았어요 (jobasseoyo). The irregular classes are lesson 17 - what matters here is that the irregularity happens before the ㅆ is attached, so the past never fixes a stem you conjugated wrongly.

Step 7 - Negation in the past

Negation is built exactly as in lesson 14; only the ending goes past.

• Short form - 안 and 못 stay free adverbs with a space, and the past marker goes on the verb: 안 갔어요 (an gasseoyo, didn’t go), 못 갔어요 (mot gasseoyo, couldn’t go).
• Long form - the past marker goes on the AUXILIARY, not on the main stem: 가지 않았어요 (gaji anasseoyo), 먹지 못했어요 (meokji motaesseoyo). Never *갔지 않아요.

Step 8 - -았/었었- for a discontinued past

Doubling the marker gives -았었-/-었었-, which says the situation held once and is now clearly over, with no trace left in the present.

• 서울에 살았어요. (Seoure sarasseoyo.) - I lived in Seoul (neutral; possibly still there).
• 서울에 살았었어요. (Seoure sarasseosseoyo.) - I used to live in Seoul (and I no longer do).

It is far rarer than English used to. Plain -았/었- is the default; reach for -았었- only when the break with the present is the point.

Step 9 - Korean past is not the English perfect

One Korean form covers I ate, I have eaten and I had eaten. So 아침 먹었어요? (Achim meogeosseoyo?) means Have you had breakfast?, not Did you eat breakfast (at some point in your life)?.

The reverse trap matters more: several Korean verbs describe entering a state, so their PAST is the natural way to say a PRESENT English state - 결혼했어요 (gyeolhonhaesseoyo) is I am married, 왔어요 (wasseoyo) is he’s here, 늦었어요 (neujeosseoyo) is I’m late.

Forms & patterns

Choosing the marker - the lesson-10 rule with ㅆ on top:

Vowel-final stems - the contractions from lesson 10, obligatory here too:

이다, 아니다, 있다/없다 and adjectives:

The past in negation, in 합쇼체, and doubled:

Examples

Common mistakes

• 어제 학교에 가요. (Eoje hakgyoe gayo.) -> 어제 학교에 갔어요. (Eoje hakgyoe gasseoyo.) - A time word like 어제 does not mark tense. Korean puts the past on the verb, every time, even when the adverb already says it.
• 밥을 먹었아요. (Babeul meogeosayo.) -> 밥을 먹었어요. (Babeul meogeosseoyo.) - After the past marker the ending is always 어요. The 아/어 choice is spent on 았/었 and is never made twice.
• 저는 학생이였어요. (Jeoneun haksaengiyeosseoyo.) -> 저는 학생이었어요. (Jeoneun haksaengieosseoyo.) - 학생 ends in a 받침, so the copula past is 이었어요. 였어요 appears only after a vowel, as in 의사였어요.
• 어제 못갔어요. (Eoje motgasseoyo.) -> 어제 못 갔어요. (Eoje mot gasseoyo.) - 못 is a free adverb and takes a space. Written solid, 못하다 is a different word meaning to be bad at something.
• 학교에 갔지 않아요. (Hakgyoe gatji anayo.) -> 학교에 가지 않았어요. (Hakgyoe gaji anasseoyo.) - In the long negative the tense goes on the auxiliary 않다, not on the main stem. The main verb stays bare: 가지.

Tips

• Build it from 해요체, not from the dictionary. Say the polite present out loud, drop 요, add ㅆ어요: 마셔요 -> 마셨어요, 돼요 -> 됐어요. One move, no new rule - and it keeps your contractions honest, because a wrong 해요체 form is the only way to get a wrong past.
• One Korean past = three English tenses. I ate, I have eaten and I had eaten are all 먹었어요. Stop looking for a perfect; Korean marks when with adverbs (어제, 아까, 작년에) and leaves the verb alone.
• Beware the state verbs. 결혼했어요, 왔어요, 늦었어요 and 알았어요 look past but describe now: I’m married, he’s here, I’m late, got it. Translating them as English pasts is one of the most common beginner misreadings.
• -았었- is not English used to. Plain -았/었- already handles most past narration. Save the doubled form for a state that is emphatically finished, and expect to hear it far less often than you would say used to.