The plural form of most nouns

The plural form of most nouns is created simply by adding the letter s.

• more than one snake = snakes
• more than one ski = skis
• more than one Barrymore = Barrymores
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Words that end in -ch, x, s or s-like sounds, however, will require an -es for the plural:

• more than one witch = witches
• more than one box = boxes
• more than one gas = gases
• more than one bus = buses
• more than one kiss = kisses
• more than one Jones = Joneses

Note that some dictionaries list "busses" as an acceptable plural for "bus." Presumably, this is because the plural "buses" looks like it ought to rhyme with the plural of "fuse," which is "fuses." "Buses" is still listed as the preferable plural form. "Busses" is the plural, of course, for "buss," a seldom used word for "kiss."

There are several nouns that have irregular plural forms. Plurals formed in this way are sometimes called mutated (or mutating) plurals.

• more than one child = children
• more than one woman = women
• more than one man = men
• more than one person = people
• more than one goose = geese
• more than one mouse = mice
• more than one barracks = barracks
• more than one deer = deer

And, finally, there are nouns that maintain their Latin or Greek form in the plural. (See media and data and alumni, below.)

• more than one nucleus = nuclei
• more than one syllabus = syllabi
• more than one focus = foci
• more than one fungus = fungi
• more than one cactus = cacti (cactuses is acceptable)
• more than one thesis = theses
• more than one crisis = crises*
• more than one phenomenon = phenomena
• more than one index = indices (indexes is acceptable)
• more than one appendix = appendices (appendixes is acceptable) 

MORE INFO:  http://grammar.ccc.commnet.edu/grammar/plurals.htm#irregular