The best replacement for must is should.

The best alternatives to must are rated as following:
★★★★★  should
★★★★★  shall
★★★★  have to
★★★  most
★★  have to exercises
★★  may
★  might
★  musk
★  have to grammar
★  need
★  will
  shall requirements
  ought
  should grammar
  should exercises
  have
  had to
  must be
  need to

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Speaking about must, one probably shouldn't forget about question.

Other search terms related to must and their ratings are:
★★★★★ question
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Google Trends

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