imperative vs must

imperative

noun
  • An essential action, a must: something which is imperative. 

  • A verb in imperative mood. 

  • The grammatical mood expressing an order (see jussive). In English, the imperative form of a verb is the same as that of the bare infinitive. 

adj
  • Having semantics that incorporates mutable variables. 

  • Expressing a command; authoritatively or absolutely directive. 

  • Essential; crucial; extremely important. 

  • Of, or relating to the imperative mood. 

must

noun
  • Something that is mandatory or required. 

  • Something that exhibits the property of being stale or musty. 

  • The property of being stale or musty. 

  • Fruit juice that will ferment or has fermented, usually from grapes. 

verb
  • To do as a requirement; indicates that the sentence subject is required as an imperative or directive to execute the sentence predicate, with failure to do so resulting in a failure or negative consequence. 

  • To become musty. 

  • Used to indicate that something that is very likely, probable, or certain to be true. 

  • To do with certainty; indicates that the speaker is certain that the subject will have executed the predicate. 

  • To make musty. 

How often have the words imperative and must occurred in a corpus of books? (source: Google Ngram Viewer )