Saying how long an action lasts
You already know how to say WHEN something happened (昨天, 今天早上 — time-when, lesson 7) and how to mark a completed action (了 — lesson 11). This lesson is about HOW LONG. Mandarin treats duration very differently from English: the duration phrase doesn't sit at the end like English 'for three years' — it goes AFTER the verb, in the object slot. And when there's also a real object, the verb gets duplicated. 我学中文学了三年。 = I've studied Chinese for three years. To ask 'how long?' you use 多长时间? duō cháng shíjiān or the more colloquial 多久? duō jiǔ. Don't confuse this with telling time on the clock — that's a totally different thing.
Duration goes after the verb, NOT at the end
The basic frame is subject + verb + 了 + duration. 我等了一个小时。 = I waited for an hour. 她睡了八个小时。 = She slept for eight hours. 我们走了三天。 = We walked for three days. The duration phrase fills what would normally be the object slot. NEVER put it at the end English-style — 我等一个小时了 would be a different (sentence-final 了) sentence about the present moment, and 我等了一个小时饭 is just broken. Common duration words: 一个小时 (an hour), 半个小时 (half an hour), 三天 (three days), 一年 (a year), 一会儿 (a little while).
The duplicated-verb pattern — when there IS an object
If your sentence has both an object AND a duration, you can't fit both in the same slot. The fix: say the verb twice. Pattern: subject + verb + object, + verb + 了 + duration. 我学中文学了三年。 = I've studied Chinese for three years. 她看书看了一个小时。 = She read for an hour. 我等他等了半天。 = I waited for him for ages. The first verb takes its object normally; the second verb takes the duration. An alternative pattern uses 的: 我学了三年的中文。 Both are common; the duplicated form is more spoken, the 的 form a bit more written. Either is fine.
Asking 'how long?' — 多长时间 / 多久
多长时间 literally = 'how-long-time' — the everyday 'how long?'. 你学了多长时间中文? = How long have you studied Chinese? 你住了多长时间? = How long have you lived (here)? 多久 is the same idea, slightly more conversational: 你来了多久? = How long have you been here? Note the question word sits in the duration slot — the same place a real number would sit. Don't tack 吗 on the end (it's already a wh-question). Compare to clock time: 现在几点? = What time is it (on the clock)? — a totally different question.
Time-of-day vs duration — the contrast
This is the crux of the lesson. 三点 = 3 o'clock (a point on the clock, lesson 7). 三个小时 = three hours (a stretch of time). 我三点来。 = I'll come at 3. 我来了三个小时。 = I've been here for three hours. The measure word 个 is a giveaway — duration of hours always uses 个小时, while clock-time uses bare 点. Days follow the same logic: 三号 = the 3rd (date) vs. 三天 = three days (duration, no measure word for 天 because it's already its own counter).
Ongoing vs. finished duration
A duration sentence with 了 alone often means 'has been doing it [and may still be]': 我学中文学了三年。 could mean either 'I studied for three years (and stopped)' OR 'I've been studying for three years (and I still am)' — context decides. To force the 'still ongoing' reading, add a sentence-final 了: 我学中文学了三年了。 = I've been studying Chinese for three years (and counting). The pair of 了s is the standard 'as of now / so far' frame. To emphasize 'I did it for three years and finished', add 完 or other context: 我学了三年中文,然后不学了。
