Most words: Add s (students, professors, buildings).
After a hard ch: Add only s (monarchs).
Words ending in ch, s, sh, ss, x and z: Add es (benches, dresses, boxes).
Words ending in is: Change is to es (thesis/theses; crisis/crises; parenthesis/parentheses).
Words ending in y: If y is preceded by a consonant or qu, change y to i and add es(babies, cities, synergies). Common nouns ending in y preceded by a vowel take only the s(alloys, days, attorneys).
Words ending in o: If o is preceded by a consonant, most plurals require es(tomatoes, echoes). Pianos is an exception. Words ending in o, directly after a vowel, take the s (folios, radios).
Some words ending in f: Add s (briefs, reefs, roofs). Other words ending in f have irregular plurals with ves(hooves, shelves, lives).
Proper Names: The plurals of most proper names are formed by adding s ( the Greens, the Browns, the McDermotts). If the name ends in s or z, form the plural by adding es (the Rosses, the Rodriguezes, the Charleses). In forming plurals of proper names ending in y, ordinarily keep the y(the Kennedys, the Lowrys and the Bradys). Some exceptions are: Alleghenies and Rockies).
Some words are the same in the plural as in the singular:chassis, corps, deer, moose, sheep).
Latin endings: Latin-root words ending in us change us to i: alumnus, alumni. Most ending in a change to ae: alumna, alumnae (formula and formulas make for an exception). Most ending in um add s (memorandums, referendums, stadiums).
Plural nouns not ending in s: Add ’s (the alumni’s newsletter, the children’s playground).
Plural nouns ending in s: Add only an apostrophe (the girls’ locker room, the students’ newspaper).
Singular nouns not ending in s: Add ’s (the book’s pages, the pen’s ink).
Singular common nouns ending in s: Add ’s unless the next word begins with s (the witness’s chair, the witness’ seat).
If a singular proper noun ends in s: Add an apostrophe (The University of Texas at Dallas’ campus).
Pronouns: Personal interrogative and relative pronouns have several forms for the possessives that do not involve an apostrophe (mine, ours, your, yours, his, hers, its, theirs, whose). If you are using an apostrophe with a pronoun, make sure that the meaning calls for a contraction: (you’re, it’s, there’s, who’s).