什么是“two-year-old”？
“two-year-old”是一个带有连字符的复合形容词，用于描述年龄为两岁的人或物。它通常放在名词前作定语，例如：
- a two-year-old child（一个两岁的孩子）
- a two-year-old dog（一只两岁的狗）
何时使用连字符？
当多个词组合起来修饰一个名词，并且位于该名词之前时，需要用连字符连接，构成一个整体形容词。例如：
- ✅ She has a two-year-old son.
- ❌ She has a two year old son.（缺少连字符，不符合规范）
但若这些词出现在名词之后作表语，则不需要连字符：
- ✅ Her son is two years old.
- ❌ Her son is two-years-old.（错误用法）
常见错误与注意事项
- 数词变化：作定语时，“year”用单数；作表语时，“year”用复数（如 two years old）。
- 不可拆分：“two-year-old”作为一个整体，不能随意去掉连字符或改变词序。
- 泛指 vs 特指：可作名词使用，如 “The two-year-old cried.”（那个两岁的小孩哭了。）
更多示例
- They adopted a three-year-old girl.
- The car is only one year old.
- As a five-year-old, he could already read.