be required to vs must

be required to

must

noun
  • 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. 

  • Something that is mandatory or required. 

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 be required to and must occurred in a corpus of books? (source: Google Ngram Viewer )