그때는 그랬지 — That's how it was then — memories, proverbs and the way things go

그때는 그랬지 — That's how it was then — memories, proverbs and the way things go. Lesson 6 of Level 4A in the haru Korean regular course.

What you will be able to do

Tell someone what you saw or found out back then, in a calm, grown-up way, with -더군(요): 학교에 가 보니 학생이 열두 명뿐이더군요. · 가 보니 공연이 벌써 끝났더군요.
Give one reason and hint that there are others, with -고 해서: 주말이고 해서 유나네 집에 놀러 갔어요. · 날씨도 좋고 해서 산책을 했어요. · 오늘은 좀 피곤하고 해서요.
Set what you heard against what really happened with -다더니, also after a saying: 일기 예보에서 따뜻하다더니 너무 춥네요. · 시작이 반이라더니 벌써 4급이에요.
Say how things usually go with -기 마련이다, and use five proverbs and V-다 보니(까): 처음에는 누구나 실수하기 마련이에요. · 매일 이야기하다 보니까 한국어가 늘었어요.

A/V-더군(요) · N(이)더군요 — I saw that … · it turned out … — telling now what you saw back then
A/V-고 해서 — … and so on, so … — one reason among several
A-다더니 · V-ㄴ/는다더니 · N(이)라더니 — they said … and (sure enough / but) … — what you heard against what really happened
A/V-기 마련이다 — … is bound to … · that's how it goes — the way things usually are