Sentence Choice – ‘Taking Day Off Monday’ vs. ‘Taking Day Off on Monday’

The grammatically correct sentence of your three examples is "Everything in that shop is on sale."

"That shop sells everything off" does not sound correct, and "That shop sells everything at discount" would actually need to be "That shop sells everything at a discount."

However, there is subtle meaning in these sentences. If you said "Everything in that shop is on sale.", it implies that the sale is temporary. A regular store would periodically put everything on sale, but will return to regular prices when it's over. (Or if a store is going out of business, saying everything in that shop is on sale is reasonable because it is marked unusually low to get rid of it.)

"That shop sells everything at a discount." implies a permanent discount. Meaning a store that tries to purposely mark everything lower than their competitors. For example, Wal-Mart sells everything at a discount.

You can also describe these types of stores as a noun, "a discount store", as in "Dollarama is a discount store."

1) One student has one book = 'open your book'. (easy) 2) One student has many books = 'open your books'. (easy) 3) Many students have one book each = ???? 4) Many students have many books each = 'open your books' (easy)

In situation 3), we can say both 'open your book' (because each student has one book each) or 'open your books' (because there are many books in total), and there's not much to choose between them. Google Ngram Viewer doesn't help. I don't think there's a answer on this one. I would probably say 'open your books', because there are many books.