Each one in turn: từng

Từng before a noun means "each one, one at a time": you go through them individually, in turn. Từng người is each person, one by one; từng phòng is room after room. With một after the noun it means "one at a time": Vào từng người một nha. Come in one at a time.

It's how you say step by step, word by word, day by day: từng bước, từng chữ, từng ngày. Cô giáo hỏi từng bạn. The teacher asked each student in turn.

Mỗi also means "each", but it shares something out or says how much each one gets: mỗi người một cái (one each). Từng is about going through them, one after another, and often about care or detail: Chị Lan kiểm tra từng tờ. Lan checked every single sheet. Mọi takes them all together at once.

Before a verb, từng is a different word: đã từng means "have ever", and chưa từng "have never": Em chưa từng đi Huế. I've never been to Huế. That one has its own page. Before a noun, it's always "each one".