Commas and periods go inside quotes:
OK: He said this is the “best possible scenario.”
NEVER: He said this is the “best possible scenario”.
OK: This is the “best possible scenario,” if you believe in such things.
NEVER: This is the “best possible scenario”, if you believe in such things.
Question marks can be tricky.
OK: Did he say this is the “best possible scenario”?
NEVER: Did he say this is the “best possible scenario?”
OK: He said, “Is this the best possible scenario?”
NEVER: He said, “Is this the best possible scenario”?
Periods with parentheses:
OK: The dog is friendly. (Even on Fridays.)
OK: The dog is friendly (even on Fridays).
NEVER: The dog is friendly (even on Fridays.)
NEVER: The dog is friendly. (Even on Fridays).
Semi colons:
You can use them in academic or expository writing (like this); but in copy, they slow things down. Try shorter sentences, commas or, for a visual break, ellipses.
CORRECT BUT NOT GOOD: WuddaCleaner is great for removing ground-in dirt; for making dogs, cats and gerbils smell springtime fresh; and it tastes great on cereal.
BETTER: WuddaCleaner is great for removing ground-in dirt … for making dogs, cats and gerbils smell springtime fresh … and it tastes great on cereal.
Run-ons and commas:
Generally, if you have two complete sentences separated by a comma without a conjunction (like and, so, but or yet), you have a run-on:
WRONG: Zest leaves no soap film, you’ll feel cleaner than ever.
You can fix it by breaking it into two sentences:
OK: Zest leaves no soap film. You’ll feel cleaner than ever.
Or by inserting a conjunction:
OK: Zest leaves no soap film, so you’ll feel cleaner than ever.
OK: Zest leaves no soap film, you’ll feel cleaner than ever, and fleas will no longer congregate in your general area.
Sometimes writers use commas where they’d take a breath when speaking out loud. It doesn’t always work in print.
WRONG: This, is the saddest day of my life.
OK: This is the saddest day of my life.
OK: This, without doubt, is the saddest day of my life.
OK: “This,” he said, choking back a sob, “is the saddest day of my life.”
OK: This is the saddest day of my life.
BIGGEST RULE OF ALL: Punctuate for clarity. Even if you have to break a rule to do it.