This reference site provides within-sentence punctuation. This site accompanies the Praxis Core Reading Test.

Clear lesson on when to use (and when not to use) a semicolon. The article ends with a self-test and answers. L.9-10.2a Punc/Indep Clause

Is it it's or its? Learn the basics about apostrophes here!

Very clear, detailed explanation of the appropriate use of colons in sentences. You will find several examples of both correct and incorrect usage, and a short quiz to test understanding. L.9-10.2b Colons