your english teacher did teach this? the grammar nazi of grammar nazis?
A semicolon is used to connect to dependent claus' (or is a depend to an independent?) when a Conjunction (For,And, Nor, But, Or Yet) wont sound right...i believe.
a colon is typically used before a list ex. "The boy carried three things in his bag
: His Toothbrush, Books and a flashlight."
a dash is used as a "Detour" as my english teacher says. To add in some info that wouldnt otherwise be needed or wanted.
to the true grammar nazis..rip me apart on the Semi colon and dash. I cant explain it in writing, but i know how theyre used. odd, i know.