First word of a sentence
Always capitalize the first word in a sentence.
Example: She loves to read.
The pronoun "I"
Always capitalized, no matter where it appears.
Example: He and I are going.
Proper nouns (names of specific people, places, organizations, and things)
Examples: Maria, London, Google, The Great Gatsby
Titles of people
Capitalize when used with a name or as a form of address: President Lincoln, Doctor Smith.
Not capitalized when used generically: The president gave a speech.
Days, months, and holidays
Always capitalized: Monday, April, Christmas. Seasons are not: spring, summer, fall.
Nationalities, languages, and religions
Examples: French, Spanish, Christianity.
Titles of works (books, movies, etc.)
Capitalize major words: nouns, verbs, adjectives, adverbs. Don't capitalize short articles, conjunctions, or prepositions unless they're the first or last word.
Example: The Lord of the Rings
First word in a quotation
If it's a full sentence, capitalize the first word: He said, "Let's go." If it's a partial quote, no capitalization: He called it "a big mistake."
Historical events and time periods
Examples: the Renaissance, World War II
Brand names and trademarks
Use the official capitalization: iPhone, Coca-Cola, eBay