※ 引述《thebeginning (當我們宅在一起)》之銘言：
[On] What day do you have PE class?
問者認為
1) 體育課只有一天，或
2) 體育課到底有幾天其實與他無關
例一：
What day do you have PE class?
Tuesdays and Thursdays.
Oh, mine's on Monday.
[問者自己體育課只有一天，所以假設答者也只有一天]
例二：
What day do you have PE class?
Tuesdays and Thursdays.
Let's have the birthday party on Monday, then.
[問者只是要確定體育課是否會與派對撞期，其實並不關心體育課到底有幾天]
[On] What days do you have PE class?
問者認為
1) 體育課不只一天
2) 體育課具重複性
例一：
What days do you have PE class?
Tuesdays and Thursdays.
Ahh, mine too, maybe we'll have PE together!
例二：
What days do you have PE class?
Tuesdays and Thursdays.
We can't have our reading group on Tuesdays, then.
單複數代表重複性在下面的例子最明顯：
親戚：When's your surgery?
病患：Monday.
護士：When's your surgery?
實習醫師：Mondays.
--
'Cause it's a bittersweet symphony, this life
Trying to make ends meet, You're a slave to money then you die
-Bittersweet Symphony, The Verve
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