1
The broken umbrella is useless.
To express this, first establish the subject by signing UMBRELLA and BROKEN. Then, emphasize its lack of value by signing USELESS, making sure to start with your 'F' handshapes and pop them open into '5' handshapes as you drop your hands outward.
2
It is useless to worry about it.
Start by signing WORRY to establish the action. Follow it up with USELESS, using a dismissive downward and outward motion while opening your hands. Adding a slight head shake will help emphasize that worrying won't change the outcome.
3
I felt useless because I couldn't help.
After signing that you were unable to help, use the sign for USELESS to describe your feelings. Opening your 'F' hands into '5' hands while dropping them outward perfectly captures the emotional weight of feeling like you have nothing to offer.