1
This cake is very sweet.
Sign "CAKE", then sign "SWEET" by starting with your open "B" handshape facing your body, curling into an "A" handshape while lowering it, and repeating. Add a pleasant facial expression to emphasize the delicious taste.
2
She is a sweet person.
Point to the person to establish the subject, then use the "SWEET" sign. The motion of curling your fingers into an "A" handshape works perfectly for both sweet-tasting food and a kind, sweet personality.
3
Do you want sugar in your coffee?
Sign "COFFEE", then sign "SUGAR" using the exact same motion as "SWEET" (curling the open "B" hand into an "A" handshape twice), followed by the sign for "WANT" and an inquiring facial expression.