a. They treat one badly over there.
b. You can't expect one to know all that.
c. You can't trust one these days.
Is 'one' used correctly in these sentences?
It is, but in different senses. "One" can mean "one of <some group>", or it can be a personal pronoun roughly translated as "me but in a wider sense including other people like me".Originally Posted by therose
In a, the second sense is the only possible one. It could be translated as "They treat people badly over there."
In b, either is possible, depending on context. For example:
"They say they taught apes to speak human language, but you can't expect one (of those apes) to know all that." - meaning 1.
"I was told to curtsey and behave like a lady, but you can't expect one (someone like me) to know all that." - meaning 2.
In c, meaning 1 is by far the most likely, but meaning 2 is just about possible. For example:
"I was taught to respect teachers, but you can't trust one (of those teachers) these days." - meaning 1.
"I used to be trustworthy, but you can't trust one (me) these days." - meaning 2. This would be a very affected form of speech.