For a start DONK, to introduce a clarification, which i take to be synonymous with “If a sentence is dragging on a bit…” you use a colon.
I ate loads for tea: chicken and mash and carrots and peas and a litre of gravy.
Colon for a tabulated list.
Colon before a direct quotation where the quotation clarifies the point.
She said the last thing I wanted to her: ‘tomorrow we start the diet’.
An introductory line before more than three lines of quoted text.
My favourite toast must be this:
blah blah blah
something Shakespearean
thus unto all ye
You can use a semicolon to join two clauses which themselves could stand as complete sentences. Usually used to balance or contrast the thrust of the two clauses. Usually use a comma before a conjunction except before the following which should be preceeded by a colon:
however
therefore
meanwhile
also
consequently
nevertheless
and a couple of others.
Semicolons ca also be used in lists to mark of items in a list.
tl;dr
If in doubt, use a semicolon and explain it away as a typo.