a. I don't know if five of my friends are still alive or not. b. I am not sure that five of my friends are still alive. Aren't these sentences ambiguous? I see two possibilities ...
For example, All that glitters is not gold is sort of a fixed term, and people often use the same “all . . . not” form when talking about things. See also the question “Is it wrong to use ‘not’ in ...
a. I did not think he would steal some of my ideas. b. I did not think he would steal certain of my ideas. Could these sentences have two meanings: I did not think he would steal any of my ideas. ...