什么是 “two years old”？
“Two years old” 是一个描述年龄的短语，通常用于句子的谓语部分，表示某人或某物的年龄。
- My daughter is two years old.
- The building is only two years old.
注意：这里的 “years” 是复数形式，因为是完整句中的表语结构。
什么是 “two-year-old”？
“Two-year-old” 是一个复合形容词（hyphenated compound adjective），用于修饰名词，放在名词前作定语。
- I have a two-year-old son.
- She bought a two-year-old car.
注意：在复合形容词中，单位名词（如 year）使用单数形式，并用连字符连接。
常见错误与对比
| 错误用法 | 正确用法 |
|---|---|
| a two years old child | a two-year-old child |
| He is two-year-old. | He is two years old. |
记忆小技巧
- 如果后面直接跟名词（如 child, dog, house），就用带连字符的 two-year-old。
- 如果是在 “be” 动词后说明年龄，就用 two years old。
- 复合形容词中，单位永远用单数！