一、year old与years old：表语用法
单独使用时，需放在be动词后作表语，用于说明主语的年龄。
year old用于基数词为“1”的情况：
She is one year old.她一岁了。
A baby one year old can walk slowly.一岁的婴儿能慢慢走路。
years old用于基数词大于“1”的情况：
My brother is five years old.我弟弟五岁了。
They have a daughter ten years old.他们有一个十岁的女儿。
二、year-old与years-old：定语用法
加连字符后作定语，用于修饰名词，需置于名词前。
year-old：基数词为“1”时，用单数形式
a one-year-old girl一个一岁的女孩
The one-year-old cat likes milk.这只一岁的猫喜欢牛奶。
years-old：错误表达！基数词大于1时，连字符内year不可加s，正确形式为“基数词-year-old”
❌ a five-years-old boy
✅ a five-year-old boy一个五岁的男孩
✅ She bought a three-year-old laptop.她买了一台用了三年的笔记本电脑。
三、核心区别
| 结构 | 语法功能 | 适用场景 | 例 句 |
|--------------|----------------|---------------------------|--------------------------------|
| year old | 表语 | 基数词=1 | He is one year old. |
| years old| 表语 | 基数词>1 | She is twenty years old. |
| year-old | 定语连字符 | 基数词=1，修饰名词 | a one-year-old dog |
| years-old| ❌错误结构 | 不可使用 | —— |
四、常见错误对比
1. ❌ The girl is five year old.
✅ The girl is five years old.表语，基数词>1用years
2. ❌ I have a 3-years-old sister.
✅ I have a 3-year-old sister.定语，连字符内year用单数
3. ❌ He is a seven year old boy.
✅ He is a seven-year-old boy.定语需加连字符
✅ He is seven years old.表语需连字符
通过以上规则，可清晰区分四者用法：表语用year(s) old，定语用“基数词-year-old”，连字符内year永远为单数。
三、核心区别
| 结构 | 语法功能 | 适用场景 | 例 句 | |--------------|----------------|---------------------------|--------------------------------| | year old | 表语 | 基数词=1 | He is one year old. | | years old| 表语 | 基数词>1 | She is twenty years old. | | year-old | 定语连字符 | 基数词=1，修饰名词 | a one-year-old dog | | years-old| ❌错误结构 | 不可使用 | —— |四、常见错误对比
1. ❌ The girl is five year old. ✅ The girl is five years old.表语，基数词>1用years2. ❌ I have a 3-years-old sister. ✅ I have a 3-year-old sister.定语，连字符内year用单数
3. ❌ He is a seven year old boy. ✅ He is a seven-year-old boy.定语需加连字符 ✅ He is seven years old.表语需连字符
通过以上规则，可清晰区分四者用法：表语用year(s) old，定语用“基数词-year-old”，连字符内year永远为单数。