Why do u say 갔어 instead of 갔아?

Question

Answer

The rule is:
어간의 끝 음절 모음이 ‘ㅏ, ㅗ’일 때에는 어미를 ‘-아’로 적고, 그 밖의 모음일 때에는 ‘-어’로 적는다.
If the final vowel of the word stem is ㅏ or ㅗ, 아 is attached. Otherwise, 어 is attached.

The final vowel of the word stem of 가(다)  is ㅏ, so 아 is attached.
가(다) + 아 => 가
The final vowel of the word stem of 먹(다)  is ㅓ, so 어 is attached.
먹(다) + 어 => 먹어

In 갔(다), 았 is not a word stem but a suffix. 
Because 았 is not a word stem, the rule is not applied. Only 어 is attached.

가 + 았 + 어 => 갔어
먹 + 었 + 어 => 먹었어

Answer

you mean, why do u say 갔어 instead of 갔다 ?

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