How was one of the nazgûl able to stab Frodo when he put the ring on at weathertop? Considering that people who put the ring on are usually invisible.
To mortal people, yes. But the Nazgûl can sense when someone is using it. Note how they immediately changed directions when Frodo put it on at the Prancing Pony.
Answer: Because they were trying to kill him and take the One Ring for Sauron.
Why didn't the nazgûl simply take ring from Frodo?
They tried, but they were interrupted by Aragorn.
lionhead
How was one of the nazgûl able to stab Frodo when he put the ring on at weathertop? Considering that people who put the ring on are usually invisible.
To mortal people, yes. But the Nazgûl can sense when someone is using it. Note how they immediately changed directions when Frodo put it on at the Prancing Pony.
Brian Katcher