How long goes after the verb: học hai năm, no 'for'

A length of time goes after the verb, with no word for "for": Tôi ở Đà Lạt ba ngày. I stayed in Đà Lạt for three days. If the verb has an object, the length of time comes after that: học tiếng Việt hai năm, study Vietnamese for two years. You can put trong in front, ở Đà Lạt trong ba ngày, which sounds a little more careful; everyday speech leaves it out.

A point in time is freer. Hôm qua (yesterday), tuần trước (last week) or lúc bảy giờ (at seven) say when. They often go first, before the person, but can also come at the end: Tôi dậy lúc bảy giờ. I get up at seven. A length of time says how long, and it goes after the verb. One sentence can have both: Tuần trước tôi ở Đà Lạt ba ngày.

To ask how long, put bao lâu (how long) where the answer goes: Anh ở Sài Gòn bao lâu? How long are you staying in Saigon? For hours, people usually say tiếng: Em ngủ mấy tiếng? How many hours do you sleep? Giờ on its own usually means the clock time (mấy giờ, what time); for a length of time you'll mostly see it in writing or as giờ đồng hồ.

For something that started in the past and is still going on, add rồi at the end, often with được before the length of time: Tôi ở đây (được) ba tháng rồi. I've been here for three months.