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MUST in sign language
How to sign "must" in American Sign Language:
Definition: To be obliged to; to be required or compelled to.
Pronunciation (sign description): Dominant clawed forefinger (handshape) held in space (location), palm down (palm orientation), moves downwards once at the wrist with a strong intonation (movement) with the mouth morpheme (NMS).
ASL Learner Tip: Not to be confused with SHOULD, HAVE-TO (kind of more euphemism than MUST).
Beyond basics
Language play is often used by Deaf signers.
Inflection and language play: The meaning of this ASL sign is the same except that it's five times MUST. That is, absolutely must.
Another context in capture (April 2020): Marcus Bryant signed MUST, double-MUST, 5x-MUST when talking about wearing a mask as absolutely required for himself before entering a chain store.
The technique of this language play is the same true for REQUIRED.
Usage/Grammar
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Written ASL
[Note: ASL writing is not an official standard. This sign language writing remains in a state of open space to allow room for experiment, evolution, and improvement.]

Contributed by ASLwrite, 2016 for "must".

Instead of using one finger, it uses the whole fingers to mean 'very must' or 'must!!!' as sometimes seen in spoken ASL.
Jolanta experimented two different handshapes (the older digit on the left and newer digit on the right) along with the mouth morpheme (intense). Below the digits is the fingerspelled word, "MUUUUST".
Contributed by Jolanta in the ASLwrite community, 2017.