It's 'useless' trying to change her mind. = It's 'useless' to try to change her mind.
Drugs are 'useless' in treating the condition.
I made a 'useless' attempt at fixing the leak.
(..) You saw what happened to Walder Frey and his family. I heard. - How could we ever trust a man like that? - We couldn't. He was a useless, old coward, but the Freys supported us. Now they're all dead. Whoever killed them is no friend of ours. We need allies. (..)2017 TvShow: Game of Thrones Title: Dragonstone Season: 7 Episode 1