不定代名詞指的是不特定的人、地方或事物。在英語中，有些不定代名詞是用量詞或個別詞形成，這類不定代名詞是以 any、some、every 和 no 開頭。
以 some 和 any 開頭的不定代名詞描述不定、不完整的量，就如單獨使用 some 和 any 的方式一樣。
|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 開頭的不定代名詞描述的是不定的量。以 every 開頭的不定代名詞是描述完整的量。而以 no 開頭的不定代名詞則是描述不存在、不在場的人或事物；通常會用在具有否定意思的肯定句，但這類句子並不是否定句，畢竟句中沒有 not 這個字。
- 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 開頭的不定代名詞可以用在肯定句，跟 every 的意思很接近：任何人、任何地方、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 開頭的不定代名詞。
- I don't have anything to eat.
- She didn't go anywhere last week.
- I can't find anyone to come with me.
在很多情況下，當句中含有 any 開頭的不定代名詞，只要將原先的不定代名詞改為以 no 開頭的不定代名詞，就可以變成帶有否定意義的肯定句。不過這樣的改寫會改變句子的意思。句中若含有 no 開頭的不定代名詞，語氣會變得更強烈，而且會傳達出防衛、絕望、憤怒等情緒。
- I don't know anything about it. = 中立
- I know nothing about it. = 防衛
- I don't have anybody to talk to. = 中立
- I have nobody to talk to. = 絕望
- There wasn't anything we could do. = 中立
- There was nothing we could do. = 防衛/憤怒
以 every、some、和 any 開頭的不定代名詞可以用在否定問句上，這類問句一般可以用 Yes/No 來回答。
以 any 和 every 開頭的不定代名詞會被用在 Yes/No 問句上；而以 some 開頭的不定代名詞，會透露出我們已知道該問題的答案、或對答案有所懷疑。
- 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 開頭的代名詞只用在問句上。通常我們會知道問句的答案，或者不會得到 Yes/No 的回答（例如在提出邀請、要求等），問這類問題的人，通常會想得到肯定的回答。
- 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? = 邀請
- 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?