Connecting Sentences · -고, -지만, -아서/어서

2. How it works

2.1 And, and then: stem + 고

Remove 다 from the verb or adjective and add 고. The ending is the same for every word. It joins two ideas of equal weight, or two actions in order.

Stem + 고   →  and / and then

먹다  →  먹 + 고  →  먹고
크다  →  크 + 고  →  크고

먹고 가요 means "eat and go". 크고 깨끗해요 means "it is big and clean". There is no spelling change at all.

2.2 But: stem + 지만

To show contrast, add 지만 to the stem. Like 고, it never changes. The first part says one thing, and the second part says something that goes the other way.

Stem + 지만   →  but

비싸다  →  비싸 + 지만  →  비싸지만

비싸지만 맛있어요 means "It is expensive, but it is delicious". The two parts do not have to be opposites. They only have to feel like a contrast.

2.3 Because, so: stem + 아서/어서

This ending has three forms. Look at the last vowel of the stem, and ignore the final consonant. After ㅏ or ㅗ use 아서. After any other vowel use 어서. For 하다 verbs use 해서.

Last vowel ㅏ / ㅗ   + 아서   가다  →  가서     오다  →  와서
Other vowels         + 어서   먹다  →  먹어서   마시다  →  마셔서
하다 verbs           → 해서   피곤하다  →  피곤해서

Vowels merge with the ending. 오 + 아서 becomes 와서, and 마시 + 어서 becomes 마셔서. For a stem that ends in ㅡ, such as 바쁘다, the ㅡ drops and the vowel before it decides: 바빠서.

2.4 Two meanings of 아서/어서

The first meaning is a reason. 바빠서 못 갔어요 means "I was busy, so I could not go". The first part is the cause and the second part is the result.

The second meaning is a sequence in which step one leads to step two. 시장에 가서 과일을 샀어요 means "I went to the market and bought fruit". You cannot buy fruit without going there, so the steps are closely linked.

바빠서 못 갔어요.         →  Reason: I was busy, so I could not go.
시장에 가서 샀어요.       →  Sequence: I went to the market and bought it.

Use 고 for a simple order of actions. Use 아서/어서 when the first action is a step toward the second. Most learners meet the "because" meaning first.

2.5 All three side by side

Here is how common words look with each ending. Notice that only the last column ever changes.

For 가다 and 만나다, the 아/어 merges completely, so you get 가서 and 만나서. For 예쁘다 the vowel before ㅡ is ㅖ, so it takes 어: 예뻐서.

2.6 Tense and what can follow

With 아서/어서, put the past tense only at the end of the whole sentence. The first part stays in the plain stem form, and the final verb tells the time for both halves. 고 and 지만 can take the past, as in 바빴지만.

아침을 먹고 학교에 갔어요.     →  I ate breakfast and went to school.
어제 바빠서 못 갔어요.         →  I was busy yesterday, so I could not go.

After 아서/어서 you cannot use a command or a suggestion. Endings such as 하세요 (please do) and 해요 as a suggestion do not fit a reason clause. Say what is true or what happened, not what the listener should do.