Writing Conventions
Sentence boundaries
The most heavily tested convention is where a sentence ends. A fragment lacks a subject, a verb, or a complete thought, as in 'Because the bus was late.' A run-on joins two independent clauses with no punctuation; a comma splice joins them with only a comma. Repair either with a period, a semicolon, or a comma plus a coordinating conjunction (and, but, so, yet, for, or, nor). A conjunctive adverb such as 'however' or 'therefore' cannot glue clauses with a comma; use a semicolon before it.
Agreement
Subject-verb agreement is decided by the true subject, not by a noun in a prepositional phrase: 'The box of tools is heavy.' Indefinite pronouns such as everyone, each, and neither are singular, and with 'neither...nor' the verb agrees with the nearer subject. Pronouns must match their antecedents in number.
Commas and apostrophes
Use commas for introductory elements, list items, and nonrestrictive information that could be removed without changing the core meaning. Do not place a single comma between two independent clauses. Apostrophes mark possession ('the team's record') and contractions ('it's' means 'it is'); 'its' is possessive. A plural noun ending in s takes the apostrophe after the s.
Parallelism and modifiers
Items in a series and after correlative conjunctions must match in form: 'hiking, swimming, and biking,' not 'hiking, swimming, and to bike.' A modifier must sit next to what it describes; a dangling modifier has nothing to attach to.
Rule of thumb
Find the main subject and its verb, ignore everything between them, and check agreement, completeness, and punctuation at the clause boundary.