名词没有单复数
Nouns have no plural

你说过“我有一本书”，也说过“我买了很多书”。两句里的“书”一模一样——它没有变长，也没有加东西。英语的 book 到了两本以上就得换样子，汉语的“书”从来不换。你已经这样说了八级了，现在给它一个名字。

You have said 我有一本书 and 我买了很多书. The word 书 is identical in both — no ending, nothing added. English book has to change shape past one; Chinese 书 never does. You have been doing this for eight levels. Now it gets a name.

所以“我看见学生了”这一句，可能是一个学生，也可能是二十个。汉语不觉得这是问题：要紧的时候，说话的人自然会说清楚。

So 我看见学生了 may mean one student or twenty. Chinese does not treat that as a gap: when the number matters, the speaker says it.

这就是量词为什么这么重要：汉语把数目的工作全交给了名词前边那一小块，名词自己一点也不管。第十一课到第十三课专门讲这一块。

This is why classifiers matter so much here. Chinese hands the whole job of counting to the little slot in front of the noun, and the noun itself takes no part in it. Lessons 11 to 13 of this level are about that slot.

“们”读轻声。它不是必须加的：“老师都来了”一样对，一样自然。加了“们”，说的常常是一群认识的、说话人心里有数的人，所以“同学们好”是对着眼前这个班说的。

们 is read in the neutral tone. It is never obligatory — 老师都来了 is just as correct and just as natural. When you do add it, you usually mean a particular group the speaker has in mind, which is why 同学们好 is addressed to the class in front of you.

为什么不能一起用？因为“们”说的是“这一群人”，不是“不止一个”。数目一说出来，就是一个一个地数；看成一个集体，就不再数了。两件事不能同时做，所以“三个学生”和“学生们”只能选一个。

Why can the two not stand together? Because 们 says 'this group of people', not 'more than one'. Say a number and you are counting one by one; add 们 and you are treating them as a single body. Chinese will not do both at once, so it is either 三个学生 or 学生们.