Can I use the word "must" in subjunctive mood?
I'm a student in China. Our English exam has a grammar item.
54．My boss ordered that the legal documents __ to him before lunch．
- A．be sent
- B．were sent
- C．were to be sent
- D．must be sent
The answer is A, but I think D is correct, too. Can I use "must" in subjunctive mood?
It's more to the point that you cannot use must in the past tense: it's had to.
In the original sentence, ordered is in the past tense, so what is ordered must either be entirely uninflected ("subjunctive") or expressed as "future-of-the-past"1.
A. My boss ordered that the legal documents be sent to him before lunch.
B. My boss ordered that the legal documents were sent to him before lunch．
C. My boss ordered that the legal documents were to be sent to him before lunch．
D. My boss ordered that the legal documents must be sent to him before lunch．
A is the uninflected "subjunctive", and is correct.
B is neither uninflected nor future-of-the-past, and is incorrect.
C is future-of-the-past, and is correct.
D is present tense, and is incorrect.
To express D in the past (as future-of-the-past) it would be "the documents had to be sent":
D1 My boss ordered that the legal documents had to be sent to him before lunch.
1 There is probably another term for expressing the future from a point of view in the past, but I don't know what it is.
As so often in this type of multiple-choice question, there is not enough information to be clear of the context, resulting in more than one possible answer:
If it means that on some occasion in the past the boss ordered you to send him documents before lunch, then A. be sent is the best answer, but C. were to be sent is also possible if the order no longer applies.
If your boss issued a new order this morning for all future documents, then both A. be sent and D. must be sent are possible - although again A. be sent is the best choice.
[If you are studying for an exam, you should of course produce the answer that is expected, based on the instruction you have received and the grammar book you are using, in this case Answer A.]