们【men】

men们 makes a pronoun or a word for people plural. It attaches to the end and takes the neutral tone.

Only for people

Apart from the pronoun tā men它们 ("they" for animals and things), 们 does not go on things. A noun like 书 has no plural form: it is "book" or "books", and the number comes from the rest of the sentence.

A number blocks 们

Even with people, 们 cannot appear once a quantity is stated, because the number already marks plurality.

• × 三本书们 → 三本书
• × 三个学生们 → 三个学生

When to leave it out

With a plain noun, 们 adds a sense of the group taken together, and it is common in address: 老师们, 朋友们, 先生们. Outside address it is left out more often than not, and 学生 alone covers both numbers.