If a word ends in s you add es (buses, horses). This rule also applies to words that end in ch, sh, ss, tch, x, or zz (couches, axes, dishes, watches).
Many words ending in y form the plural by changing the y to ies (try becomes tries, kitty becomes kitties). When the y is preceded by a vowel, you usually just add an s (days, trays). For proper names where a consonant precedes the y, just add s (Kelly becomes Kellys).
Words ending in o sometimes cause confusion. Many form the plural by simply adding s (radios, solos, pianos), but others add es (potatoes, tornadoes, tomatoes). Check a dictionary if you're uncertain.
Form the plural of an individual letter by adding 's (A's, B's, C's, D's, F's). This is the one of the only times you use an apostrophe to form a plural (it helps avoid confusion with other words). Form the plural of multiple letters or single- or multiple-digit numbers without an apostrophe: ABCs, pdfs, 1970s, 1s.
There are other ways to form plurals in English. Consult a dictionary for correct spelling of any word you're unsure of. --Linda Gilmore lindagi@ksu.edu