What you did: the past tense

The past ending is 았어요 after ㅏ and ㅗ and 었어요 after everything else, the same bright and dark vowel choice as day 13. When the stem ends in a vowel, the same merges happen too: 가 plus 았어요 becomes 갔어요, and 보 plus 았어요 becomes 봤어요. The shortcut is to take the -요 form, drop the 요, and add ㅆ어요: 가요, 가, 갔어요. When you speak, the ㅆ links into the 어 that follows, which is why you hear ga-sseo-yo.

Korean marks the past in one place, inside the verb, and nowhere else in the sentence. English spreads the work around, with irregular pasts like 'ate' and 'went' and an extra 'did' for questions, while Korean simply swaps the ending. Even 하다, the one irregular verb on day 13, is regular from there: 해요 gives 했어요. The 었 in 먹었어요 is the same piece you met in 잘 먹었습니다 on day 12, with the polite -요 ending after it instead of the formal one.

The copula from day 2 has its own past, and it keeps the same consonant and vowel split: 이에요 after a consonant becomes 이었어요, and 예요 after a vowel becomes 였어요, which is 이었어요 with the vowels squeezed together. Day 5's 아니에요 works the same way, giving 아니었어요, 'was not'. For every verb, the time word does the rest: 어제 or 주말에 at the front tells the listener which past you mean. At this stage there is no difference to worry about between 'I ate', 'I have eaten' and 'I did eat'; 먹었어요 covers all three.