Can someone tell me when to use a colon v. a semi-colon?
I honestly never know if I'm using those things right!
As I understand it, the main reason to use a semi-colon is to link two complete sentences (not clauses or phrases) that are related.
He seems to know what he's talking about; I remain unconvinced.
You can also use the semi-colon like the Oxford or serial comma when your series also requires use of a comma. The use of the semi-colon provides clarity. Like so:
Each child was seated at a separate station and given the following plush toy or toys: an elephant, which all children saw in the previous experiment; a kangaroo, which only half of the children saw in the previous experiment; or both the elephant and the kangaroo.
Note the use of the colon in the example above. As I understand them, colons are most commonly used to signal an appositive clause, where you are describing in detail whatever comes before the colon: in the example above, the colon sets off a description of what toy or toys were given.
As D.O.S. mentions, an m-dash can often serve the same purpose as a colon. I also like using m-dashes to set off clauses or phrases that are intended to be read as parenthetical interjections.
I know we were out far too late last night--and believe me, I had a great time--but I'm definitely regretting it this morning.
Note that in the above example, the m-dash isn't rendered correctly; it should be a single, longer dash the length of a capital M (hence, its name).