In the sign for "can't," both hands are facing palm down and are in "index finger handshapes." Hold the right hand a few inches above and an inch to the right of the left hand. Bring the right hand down sharply and whack the fingernail of the left index finger with the fingerprint side of the right index finger.
CAN'T:
Note: The left hand remains fairly stationary during this sign. It is the right hand that moves. Think of a mother whacking her child's finger while telling him he can't touch the stove because it is hot.
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Since you are expressing a "negative" concept, it is common to add a negative head-shake.
CAN'T:
Also see: CAN
Notes: