use of the colon:
where the second clause explains or follows from the first:
The politician is being explicit with members: He will back another vote in the new year.
Don't do it for the rewards: do it for the planet.
It's not all bad news: I hope anyway.
I may one day find it in myself to watch the remake: it looks awful.
How does this differ from a semi colon or em dash when used this way ?
None of those take a colon. OK, if you want to argue the politician one, maybe. The colon is pretty rare, and so is the semicolon.
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None of those take a colon. OK, if you want to argue the politician one, maybe. The colon is pretty rare, and so is the semicolon. Try a period and new sentence first to see whether that isn't all you need, and it almost always is.