不定代词指的是不特定的人丶地方或事物。在英语中，有些不定代词是用量词或个别词形成，这类不定代词是以 any丶some丶every 和 no 开头。
以 some 和 any 开头的不定代词描述不定丶不完整的量，就如单独使用 some 和 any 的方式一样。
|I would like to go to Paris this summer.||I would like to go somewhere this summer.|
|Jim gave me this book.||Someone gave me this book.|
|I won't tell your secret to Sam.||I won't tell your secret to anyone.|
|I bought my school supplies at the mall.||I bought everything at the mall.|
在肯定句中，以 some 开头的不定代词描述的是不定的量。以 every 开头的不定代词是描述完整的量。而以 no 开头的不定代词则是描述不存在丶不在场的人或事物；通常会用在具有否定意思的肯定句，但这类句子并不是否定句，毕竟句中没有 not 这个字。
- Everyone is sleeping in my bed.
- Someone is sleeping in my bed.
- No one is sleeping in my bed.
- I gave everything to Sally.
- He saw something in the garden.
- There is nothing to eat.
- I looked everywhere for my keys.
- Keith is looking for somewhere to live.
- There is nowhere as beautiful as Paris.
以 any 开头的不定代词可以用在肯定句，跟 every 的意思很接近：任何人丶任何地方丶任何事物。
- They can choose anything from the menu.
- You may invite anybody you want to your birthday party.
- We can go anywhere you'd like this summer.
- He would give anything to get into Oxford.
- Fido would follow you anywhere.
否定句只能用 any 开头的不定代词。
- I don't have anything to eat.
- She didn't go anywhere last week.
- I can't find anyone to come with me.
在很多情况下，当句中含有 any 开头的不定代词，只要将原先的不定代词改为以 no 开头的不定代词，就可以变成带有否定意义的肯定句。不过这样的改写会改变句子的意思。句中若含有 no 开头的不定代词，语气会变得更强烈，而且会传达出防卫丶绝望丶愤怒等情绪。
- I don't know anything about it. = 中立
- I know nothing about it. = 防卫
- I don't have anybody to talk to. = 中立
- I have nobody to talk to. = 绝望
- There wasn't anything we could do. = 中立
- There was nothing we could do. = 防卫/愤怒
以 every丶some丶和 any 开头的不定代词可以用在否定问句上，这类问句一般可以用 Yes/No 来回答。
以 any 和 every 开头的不定代词会被用在 Yes/No 问句上；而以 some 开头的不定代词，会透露出我们已知道该问题的答案丶或对答案有所怀疑。
- Is there anything to eat?
- Did you go anywhere last night?
- Is everyone here?
- Have you looked everywhere?
- Isn't there anything to eat?
- Didn't you go anywhere last night?
- Isn't everyone here?
- Haven't you looked everywhere?
以 some 开头的代词只用在问句上。通常我们会知道问句的答案，或者不会得到 Yes/No 的回答（例如在提出邀请丶要求等），问这类问题的人，通常会想得到肯定的回答。
- Are you looking for someone?
- Have you lost something?
- Are you going somewhere?
- Could somebody help me, please? = 要求
- Would you like to go somewhere this weekend? = 邀请
- Aren't you looking for someone?
- Haven't you lost something?
- Aren't you going somewhere?
- Couldn't somebody help me, please?
- Wouldn't you like to go somewhere this weekend?