Q: A sign in the bathroom of the ladies’ locker room says, “It is imperative that nothing but TP is put in the toilet.” Aside from the fact that a couple of other things also go in the toilet, shouldn’t this read “be put,” not “is put”?
A: A sentence like that is referred to as a mandative construction; it demands something. It includes a mandative adjective (“imperative”) that governs a subordinate clause expressing what’s demanded.
The two usual ways to write such a sentence are (1) “It is imperative that nothing but TP be put in the toilet” and (2) “It is imperative that nothing but TP should be put in the toilet.” A much less common and somewhat iffy version is (3) “It is imperative that nothing but TP is put in the toilet.”
The Cambridge Grammar of the English Language, by Rodney Huddleston and Geoffrey K. Pullum, says a mandative adjective can be followed by (#1) a “subjunctive mandative” clause, (#2) a “should mandative” clause, or (#3) a “covert mandative” clause. The term “covert” here describes a tensed usage with a hidden subjunctive sense.
“Clear cases of the covert construction are fairly rare,” the authors add, “and indeed in AmE are of somewhat marginal acceptability. In AmE the subjunctive is strongly favoured over the should construction, while BrE shows the opposite preference.”
The Cambridge Grammar includes many examples of the three types of mandative construction, including these: (1) “It is essential that everyone attend the meeting”; (2) “It is essential that everyone should attend the meeting”; (3) “It is essential that everyone attends the meeting.”