I read the book The Not So Short Introduction to LaTeX, and found this line:
LaTeX: Tilde '~' generates a space that cannot be enlarged and...
It seems that the tilde make the space smaller when I compare Mr.~Smith and Mr. Smith.
Question 1: When shall I generate a space that cannot be enlarged?
In addition, same in the section The Space Between Words, there is a line:
If a period follows an uppercase letter, this is not taken as a sentence ending,...
However, no matter uppercase or lowercase I follow a period, I cannot find something different.
Question 2: Can anyone show me an example?
Thirdly,
The command \@ in front of a period specifies that this period terminates a sentence even when it follows an uppercase letter.
Similarly, for those two lines, I cannot find any difference.
I am new to Latex\@. And you?
I am new to Latex. And you?

~(tilde) is mostly a fixed and non-breakable space. ->Donald~E.~Knuth– Johannes_B Jul 25 '15 at 12:05lshort? – Johannes_B Jul 25 '15 at 12:07xxx ~yyywhich is two spaces a normaland an additional~you just wantxxx~yyyif you are using~. – David Carlisle Jul 25 '15 at 12:27\frenchspacing/\nonfrenchspacing. Try to use this sitch to see the differences. – Kola B. Jul 25 '15 at 12:08