One rotten apple spoils the whole barrel.或者The rotten apple injuries its neighbours.
一只烂苹果烂一萝苹果。（一只老鼠，坏了一锅粥）
There is a small choice in rotten apples.
朽木不可雕。
Throw away the apple because of the core.
因噎废食。
upset sb.'s apple cart 破坏某人计划
compare apples and oranges = to compare two things that simply cannot be compared
比较两个无法相比的事物
as like as an apple to an oyster
毫无相同之处
That's ridiculous. Now you're comparing apples and oranges.
真可笑，你这是拿两种无法相比的东西作比较，就好像苹果和桔子。
apple-polisher 马屁精
polish the apple （美俚）拍马屁, 讨好
She is a real apple-polisher for the way she's crawling around the boss and making eyes at him. 她是一个马屁精，整天围着老板眉来眼去的。
the Big Apple = New York 大苹果城[即纽约]
I live in the Big Apple. 我住在大苹果城