CAUTION: NOT A TEACHER
(1) This is how I interpret your sentences. I do not claim that I am right.
(2) First, the "easy" one: a really good restaurant = a very good restaurant.
(3) Now let's get to yours:
(a) "There's a good restaurant really not far from here."
I feel that you are answering someone who does not believe you. It's as
if you said:
Really! There's a good restaurant not far from here.
There's a good restaurant not far from here. Really!
It's as if you had written the sentence with commas:
There's a good restaurant, really, not far from here.
(And in speech, you would use pauses for those commas.)
In other words, you want your friend to believe the truth of your statement.
(b) There's a good restaurant not really far from here.
As someone explained in another thread, the position of "really" means
"just a little bit." OK, the restaurant is one mile away, but that is not really
far. We're young people. We can walk that distance in a few minutes. And a car
would get us there even more quickly.
Compare: There is a famous man (Of course, I shall not name him) who claims that
he is fluent in French, but when French journalists asked him questions in French, he
could not understand them. He does not really know French, although he might be
able to string together a few sentences or he may be able to read it (with a dictionary
at his side) or read aloud a speech (written for him).
Another example: Flight attendants aboard international flights can say a few sentences in many languages. But most of them do not really speak those