General rules of English can get you a long way, but both the beauty and the bane of English is that there are exceptions to everything. Pluralizing words isn't always a simple matter of adding an s on the end, and switching a word from masculine form to feminine form isn't always a matter of adding -ess.

Plurals

The words alms, amends, cattle, clothes, doldrums, ides, pants, pliers, scissors, shorts, smithereens, and trousers are all plural but have no singular form.

Many words, such as deer, moose, and sheep, are spelled and pronounced the same way in both their singular and plural forms. More interesting words with this property are congeries, kudos, premises, shambles, series, and species. Fish can be both singular and plural, yet fishes is also a correct pluralization of the word.

The words bourgeois, chassis, corps, faux pas, gardebras, précis, pince-nez, and rendezvous all have plurals spelled the same way but pronounced differently.

Kine, an obsolete plural form of cow, shares no letters with its singular form.

The plural of man is men. The plural of woman is women. The plural of human is humans.

The plural of foot is feet. The plural of goosefoot is goosefoots.

The plural of moose is moose. The plural of goose is geese. The plural of mongoose is mongooses.

The plural of mouse, the rodent, is mice. The plural of mouse, the computer hardware device, is mouses.

Other unusually pluralized words are brother, which may be pluralized to brothers but also brethren; cherub, which is pluralized to cherubim; die, which is pluralized to dice; formula, which may be pluralized to formulas but also formulae; juger, which is pluralized to jugera; kibbutz, which is pluralized to kibbutzim; landsman, which is pluralized to landsleit; libretto, which is pluralized to libretti; ox, which is pluralized to oxen; paries, which is pluralized to parietes; person, which is pluralized to people; rubai, which is pluralized to rubaiyat; schema, which is pluralized to schemata; seraph, which is pluralized to seraphim; tempo, which is pluralized to tempi; and wunderkind, which is pluralized to wunderkinder. Most of these words were taken from other languages -- like Hebrew, Greek, German, and Italian -- with the foreign pluralization rules retained.

The singular form of braces, when used in the orthodontic sense, is bracket. One bracket per tooth is attached when someone gets braces.

Hair is a singular word that suggests more than its plural, hairs.

The plural words abbes, abys, adventures, bas, bos, bras, bulgines, cares, chapes, cites, cosines, deadlines, esquires, fras, gamines, gaus, glassines, gues, hos, kavas, kas, larges, las, los, lownes, marques, mas, millionaires, mis, moras, mos, multimillionaires, nervines, ogres, pas, pis, pos, posses, prelates, princes, pros, sagenes, saltines, shines, sightlines, squires, tartines, timelines, tyrranes, and usures all become different singular words if you add another s onto the end of each. Many of them switch from masculine plural form to feminine singular form.

Gender

Widower is the only word in the English language whose masculine form is longer than its feminine form, which is widow.

Demirep is the only word in the English language which is made feminine by applying a prefix, rather than a suffix, to the masculine form, which is rep.