核心结论
在绝大多数情况下，动词 spend 后面应搭配介词 on，而不是 in。
正确结构为：
- spend + 时间/金钱 + on + 名词
- spend + 时间/金钱 + (in) + 动名词（doing） —— 此处 in 可省略
详细用法解析
1. spend ... on + 名词
当表示“把时间或金钱花在某事/某物上”时，使用 on。
• I spent $50 on a new book.
• She spends a lot of time on her homework.
• She spends a lot of time on her homework.
2. spend ... (in) doing something
当后面接动词时，用动名词（-ing 形式），此时介词 in 通常省略，直接说 “spend time/money doing...”。
• He spent two hours (in) cleaning the room. → 更常说：He spent two hours cleaning the room.
• They spent all their savings (in) traveling. → 更自然：They spent all their savings traveling.
• They spent all their savings (in) traveling. → 更自然：They spent all their savings traveling.
注意：虽然语法上 “in + doing” 是成立的，但在现代英语中，in 通常被省略，除非为了强调或修辞需要。
3. 常见错误
❌ 错误：I spent money in books.
✅ 正确：I spent money on books.
❌ 错误：She spends time in watching TV.
✅ 正确：She spends time watching TV. （或 rarely: in watching TV）
记忆小技巧
记住这个口诀：
“Spend on things, spend doing actions.”
也就是说：花钱/时间在东西上用 on；花钱/时间做事情用 doing（不加 in）。
总结
- ✅ spend + 钱/时间 + on + 名词
- ✅ spend + 钱/时间 + doing（动名词）—— in 通常省略
- ❌ 不要用 spend + in + 名词