Frege, Sense and Reference

- How does the difference in the modes of presentation explain the difference between "a=b" and "a=a"?
- What is the sense of a linguistic sign?
- What is the sense of a proper name?
- Can a name have sense but no reference?
- What is the example of observing the Moon through a telescope supposed to illustrate?
- What is a the sense of a sentence? What is its reference?
- What is the reference of a sentence where one of the names occurring in it has no reference?