How long and how many times: 了 + duration, V 了三次

Time words such as 昨天 and 每天 come before the verb, but a measured stretch of time comes after it: 我睡了八个小时 “I slept for eight hours.” The same is true of a number of times: 我去过三次中国 “I’ve been to China three times.” This is the reverse of what most learners expect, and 我八个小时睡了 is the standard beginner error.

When the verb has an object, the measure and the object compete for the same slot, and Chinese solves it in one of two ways. Either put the measure in front of the object, optionally with 的 — 我学了两年（的）汉语 “I studied Chinese for two years” — or repeat the verb and hang the measure on the second copy: 我学汉语学了两年. Both are correct; the first is shorter, the second is common when the object is long.

A pronoun object behaves differently: it stays right after the verb and the measure follows it. 我等了他半个小时 “I waited half an hour for him”, never 我等了半个小时他. Names behave like pronouns in this respect.

Add a second 了 at the end and the situation is still going on: 我学了两年汉语了 “I’ve been studying Chinese for two years (and still am).” Without it, the study is finished.

The common frequency measures are worth learning together: 次 is a countable occasion, 遍 is one run through something from beginning to end (这本书我看了两遍 “I read the book twice, cover to cover”), 下 is one brief movement (敲了三下 “knocked three times”), and 趟 is one there-and-back trip.

In the negative there is no duration complement at all, because nothing happened for any length of time: 我没睡好 or 我三天没睡好觉, where the time phrase moves back in front of the verb.