Dates

Use numbers for decades, with no apostrophe separating the zero and the “s.”

1350s, 1770s, 1990s
Write out centuries. twelfth century, nineteenth century
Hyphenate the century if it functions as a compound adjective, but not if it functions as a noun. eighteenth-century carnivals, carnivals in the eighteenth century

Quotations

Place commas and periods inside the ending quotation mark.

Greta acknowledged that there were “conspicuous lacunae,” typos, and statistical errors in her “very preliminary paper.”

Place question marks outside the ending quotation mark...

Was there a way to win the debate without resorting to “strong-arm tactics”?
...unless the quoted material itself is a question. As the car approached Durham, Graham asked the driver, “Should we roll our windows up?”

Than / then
Than is used in comparative statements. The development of radio was more revolutionary THAN the development of television.
Then is used with a sequence of events. The merchants first tried a political solution and THEN turned to a religious solution.

Affect / effect
Affect is a VERB meaning "have an influence on." The donation from the industrialist will affect the Senator's vote.
Effect is a NOUN meaning "result." What effect will the industrialist's donation have on the Senator's vote?

Split infinitives
Do not split infinitives (verbs in the form "to _verb_") with adverbs.

As Evan Jenkins of the Columbia Journalism Review writes, "Splitting an infinitive is not a mortal sin, but it's nice to avoid because it makes some grammarians and other thoughtful readers grind their teeth."
NO: To better understand the miners’ mood, he went to live among them.
YES: To understand the miners’ mood better, he went to live among them.

  NO: Valery wanted to quietly walk on the ramparts.
YES: Valery wanted to walk quietly on the ramparts.


Apostrophes
It's (with an apostrophe) is the contraction of "it is." It's a shame about Ray.

Its (without an apostrophe) is a possessive pronoun (such as "his" or "her") requiring no apostrophe to show possession.

The committee published its report.
Apostrophes indicate a contraction or possession. Do not use apostrophes to pluralize!

YES: The three opponents eyed each other warily.
NO: The three opponent's eyed each other warily.

For plural possessives, place the apostrophe after the final s. YES: The three opponents' eyes were blue.
NO: The three opponent's eyes were blue.

Footnotes

Book

Pamela Scully, Liberating the Family? Gender and British Slave Emancipation in the Rural Western Cape, South Africa, 1823-1853 (Portsmouth, NH: Heinemann, 1997), 210-3.

Chapter in a book (if the chapter’s author is the book’s author)

John Bossy, “Kith and Kin,” chap. in Christianity in the West, 1400-1700 (Oxford: Oxford University Press, 1985), 23.

Chapter in a book (if the chapter’s author is not the book’s editor)

Tony Bennett, “The Exhibitionary Complex,” in Culture / Power / History: A Reader in Contemporary Social Theory, ed. Nicholas B. Dirks, Geoff Eley, and Sherry B. Ortner (Princeton: Princeton University Press, 1994), 124.

Journal article

Michael Santos, “Between Hegemony and Autonomy: Skilled Iron Workers’ Search for Identity, 1900-1930,” Labor History 35 (Summer 1994): 403-4.

Newspaper article

Omaha Telegraph, 18 July 1902.
Justice (London), 5 September 1811.

Letter

Willa Cather to James Trenton, 3 December 1895, Jaret Collection, Carroll Library, Denison University Library, Granville, Ohio.

Archival source

Carnegie Steel Company, “General Plan of Works Showing Course for Visitors to Follow through the Mill, 1907,” in William Gaughan collection, Archives of Industrial Society, University of Pittsburgh, Pittsburgh.