... just to use "present passive tense". The entire sentence can be re-written as: These desks which are left in the hall must be removed soon. Again ...
... of a flat, smooth top supported by legs 桌子；台子 Desk 1. a frame or table for reading or writing upon 書桌；寫字台 2. a table or stand to hold that from which one reads or...
stationary(=writing materials) is common collective nouns,not in eg. uncountable nouns always singular...furniture, luggage, rubbish, bread.,not in eg. B:- There are ( an eraser and a ruler) on the desk.