Question

Choose the most suitable modal verb for this sentence:
You ___ speak only when you're spoken to. (compulsion)

• A. Ought to
  
• B. Must
• C. Dare to
  
• D. Used to

Answer 1

The correct option is B Must

Modal verbs help determine the mood of a sentence, to indicate obligation, permission, possibility and so on. Semi/quasi modals are generally modal phrases performing the same function as modals, they form phrases with 'to', like ought to, need to, used to and so on. 'Must' indicates obligation or compulsion, whereas 'ought to' indicates a moral obligation, for example, 'you ought not to disrespect your teachers'. The given statement indicates a compulsion, and not a moral obligation hence option B is correct and A is incorrect. 'Dare' means 'be bold enough to' and 'used to' refers to a routine or pattern; none of these are relevant to the given sentence, the speaker neither threatens (dare to) the listener nor does he state his habit/routine (used to); hence options C and D are incorrect.