My 2 cents... Both expressions "not exactly" and "not really" you would say after someone said something to you. Let's say I am a stubborn but hard working person. Let's say somebody says to me, "I heard you are a stubborn and lazy guy." Then I would say, "not exactly... although I am stubborn I am not lazy." It works to also say "not really... although I am stubborn I am not lazy" but the first expression fits a little better. Basically they are very interchangeable, but not exactly means they are like 60% correct and not really could mean they are 0% to 100% correct. Here's another example... let's say somebody says 2+2 = 4.0001 I would say... "not exactly" But if they say 2+2 = 10,000 then I would say "not really" To be honest in both cases I would say "dude stop doing drugs" Hopefully someone can come up with a better explanation for you =) Though, I wouldn't stress about the two phrases.