1. Specifying the length of an action with a duration phrase
Duration phrases can be used to indicate HOW LONG an action occurs.
Subject + verb + duration
Open-ended actions that have duration: 买，学，看，吃，睡， 唱，洗， etc.
Example:
他在中国住了一年。He lived in China for a year.
他每天看一个钟头的报。He reads a newspaper for one hour every day.
Indicating duration when there is no object noun phrase
When the action verb does have an object noun phrase, the duration phrase simply follows the verb. If the verb is suffixed with 了， the duration phrase follows the verb 了
Example:
我想在美国住两年。I plan to live in USA for two years.
他病了三天。 He was sick for three days.
Indicating duration when the verb takes an object
The verb is followed by its object or by a duration phrase. A single verb is never followed directly by both and object and a duration phrase, with one exception.
If the object is a pronoun, the pronoun may occur after the verb and before the duration phrase.
Example:
我等了他三个小时了。
Pattern 1
Verb + object verb + duration
Example:
我每天晚上看书看三个小时。(NOT我每天晚上看书三个小时)
我睡觉睡了八个钟头。
我等他等了五个小时。
Pattern 2
Verb + object 有 + duration
Example:
我等王老师有三个小时了。 I have been waiting for Wang Laoshi for three hours.
Pattern 3
Verb + duration 的 object noun phrase
Example:
我学了两年的中文。I studied Chinese for two years.
我睡了八个钟头的觉。 I slept for eight hours.
Pattern 4
Object noun phrase, verb + duration
Example:
这个问题，我已经想了很久了。This problem, I have already thought about it for a long time.
中文， 我已经学了四年了。Chinese, I have already studied it for four years.
中文，我至少会学两年。 Chinese, at the very least I will study it for two years.
Duration vs time when
Note the difference between phrases that indicate duration and those that indicate time when.
Time phrases that indicate the duration of a situation always occur after the verb:
我学了两个小时了。 I studied for two hours
Time phrases that indicate the time when a situation takes place always occur before the verb:
我昨天学了中文。 I studied Chinese yesterday
A sentence may include a ‘time when’ and a duration phrase:
我昨天学了两个小时。 Yesterday I studied for two hours.
