在英语语法中，不定代词如"some", "any", "no", "every", "each"等放在句首构成的句子被称为不定代词句，通常用来表达疑问、否定或推测的语气。不定代词句的常见形式包括"Some/Any/No + 名词 + 动词"，"Every/Each + 名词 + 动词"和"Somebody/Anybody/Nobody/Everybody + 动词"。不定代词句中的谓语动词通常使用单数形式。
不定代词放在句首，会构成特殊的句子结构，称为不定代词句。不定代词句是以不定代词作为主语的句子，通常用来表达一种疑问、否定或推测的语气。
以下是不定代词句的几种常见形式：
1. Some/Any/No + 名词 + 动词
- Some people like cats, but others prefer dogs.
- Anybody can learn how to cook if they practice enough.
- No one knows the answer to that question.
2. Every/Each + 名词 + 动词
- Every child loves to play games.
- Each student must bring their own textbooks to class.
3. Somebody/Anybody/Nobody/Everybody + 动词
- Somebody has already taken my seat.
- Nobody likes to be criticized.
- Everybody needs a good night's sleep.
不定代词句通常用来表达询问、否定或推测的语气，因此在日常生活中使用频率较高。但是需要注意的是，不定代词句中的谓语动词通常使用单数形式，因为不定代词本身就是单数形式。
