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.
Since you are expressing a "negative" concept, it is common to add