Mechanics
The marks that change meaning
Mechanics questions cover capitalization, punctuation, apostrophes, numbers, and abbreviations. You rarely have to invent a rule - you have to notice the one being broken.
Commas do five jobs: after an introductory phrase or clause (After the audit, ...); around information that could be removed without changing the core meaning (The policy, which took effect in May, applies to everyone); between two independent clauses joined by and, but, or, so; between items in a series; and between coordinate adjectives (a clear, concise memo). Do not use a comma to glue two sentences together, and do not separate a subject from its verb.
Semicolons join two closely related independent clauses and separate series items that already contain commas. Colons must follow a complete sentence and introduce a list, example, or explanation; a colon placed directly after a verb or preposition is nearly always wrong.
Apostrophes mark possession or contraction, never simple plurals. The employees' badges (several employees) versus the employee's badge (one). Watch its (possessive) against it's (it is).
Capitalization in business writing is narrower than people assume. Capitalize proper nouns and titles used directly before a name (Director of Sales Dana Ruiz) but lowercase them standing alone (the director).
Rule of thumb: read the sentence aloud and pause where the mark sits. If you would not pause, the comma probably does not belong; if you pause but no mark exists, check for a run-on.