了 (le) is a small word with two main jobs. After a verb, it shows that an action is finished. At the end of a sentence, it shows that something has changed or is new. It is one of the first grammar points of HSK 1.
Subject + verb + 了 + object.
Put 了 straight after the verb to say the action happened and is complete. This works best when the object has a number or a describing word in front of it, such as 三本书 or 很多东西. A short object on its own, as in 我买了书, often sounds unfinished, as if more is coming.
When 了 comes at the end of the sentence, it tells the listener that the situation is new: it has started raining, it is now eight o'clock, you have changed your plans. This 了 can also follow an adjective, as in 天冷了, or a negative with 不, as in 我不想去了.
With a simple verb and object, such as 吃饭 or 睡觉, putting 了 at the end is the natural way to say it has happened.
With a length of time, you can use 了 in both places. The action began in the past and is still going on now.
To say an action did not happen, use 没有 before the verb and drop 了. A very common mistake is to keep 了 and say 他没有来了 instead of 他没有来. Use 还没有 for "not yet".
To ask whether something has happened, add 吗 after 了, or end the question with 了没有. Answer yes with the verb and 了, or no with 还没有.
Learners often add 了 to everything that happened in the past. Chinese does not need it there. If you are describing how things were, or something you did regularly, a time word such as 昨天 or 去年 is enough.
Every example on this page uses only words up to HSK 1. Paste your own sentences into the level checker to see which words, if any, are above your level.