完成体：了
Perfective 了

“我吃了饭。”“我买了三本书。”“下雨了！”“我不吃了。”这四句话你从第四级起就会说，可是里面有两个不一样的“了”：前两句的“了”跟在动词后面，后两句的“了”在句子最后。今天把它们分开来看。

我吃了饭. 我买了三本书. 下雨了! 我不吃了. You have said all four since level 4 — but there are two different 了 in them. In the first pair it follows the verb; in the second it closes the sentence. This lesson pulls them apart.

动态助词“了”的位置很死：紧跟在动词后面，宾语在它后面。要注意一点：动词带“了”的时候，宾语一般要有数量或者定语，不然这句话像还没说完。“我买了书”会让人等着下文，“我买了三本书”才是完整的一句话。

The position is fixed: 了 clamps onto the verb, and the object follows it. One habit worth building — when the verb carries 了, the object usually needs a number or a modifier, otherwise the sentence sounds unfinished. 我买了书 leaves your listener waiting for the rest; 我买了三本书 stands on its own.

这个“了”不管动词，它管整个句子：以前是一个样子，现在换了一个样子。所以它可以跟在形容词后面（天气冷了），也可以跟在名词性成分后面（他二十岁了），这些地方根本没有动作。两个“了”还能同时出现：“我吃了饭了”——饭吃完了，而且这是一个新情况。

This 了 has no interest in the verb; it reports on the whole situation: things were one way, now they are another. That is why it can follow an adjective (天气冷了) or even a noun phrase (他二十岁了), where there is no action at all. Both 了 can appear together: 我吃了饭了 — the meal is finished, and that is news.

在这种句子里，“了”说的是两件事的先后：第一件完了，第二件才开始。至于这两件事在昨天还是在明天，由时间词决定。这就是上一课那条规则最清楚的证据——“了”是体，不是时态。

In sentences like these, 了 is doing one job: event one finishes, then event two begins. Whether the pair sits in yesterday or tomorrow is settled by the time word. This is the clearest proof of last lesson's rule — 了 is aspect, not tense.

道理很简单：“没”说的就是“这件事没有实现”，“了”说的是“实现了”，两个不能放在一起。所以“我没买了书”永远是错的。要说事情没有发生，用“没”；要说习惯、打算、意愿上不做，用“不”——“我没买书”是事情没有发生，“我不买书”是不想买。

The logic is simple: 没 says the event was not realized, 了 says it was, and they cannot share a verb — 我没买了书 is never Chinese. Use 没 for an event that did not happen, 不 for habit, plan or refusal: 我没买书 means you didn't buy any, 我不买书 means you won't.