The basic rule is: add -s to most nouns. But several spelling rules change how the plural is formed.

1

Nouns ending in -s, -ss, -sh, -ch, -x, -z — add -es (this adds a syllable so you can say it). bus → buses, church → churches, box → boxes, watch → watches.

2

Nouns ending in consonant + y — change y to -ies. city → cities, baby → babies, story → stories. (But vowel + y just adds -s: day → days, boy → boys.)

3

Nouns ending in -f or -fe — change to -ves. knife → knives, leaf → leaves, half → halves. Exceptions: roof → roofs, chef → chefs.

4

Irregular plurals — must be memorised. man → men, woman → women, child → children, tooth → teeth, foot → feet, mouse → mice, person → people, sheep → sheep, fish → fish.

Uncountable nouns have no plural at all: information, advice, furniture, luggage, money, homework, news, water, music. Never add -s to these. "Two informations" and "an advice" are always wrong.