This is a large subject and you will need to put in time and effort. To begin, the general rule is to add "s" to the singular form. You will need to research the varying forms of a noun ending in 1) a consonant +Y 2) f,fe
3) a sibilant 4) O
Some plurals are formed by a change of a vowel ie man >men
by adding -n/-en ie oxen.
singular as plural > neuter plurals sheep , deer.
foreign nouns ie single "radius" plural "radii"
double plurals ie brother > brethren brothers.
I realise this is not a full answer but it should help you search for more examples in these categories.
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