common mistakes made while speaking english : peoples......there were many peoples in the market

As a general rule , the plural is formed by adding "-s " to the singular form of nouns.

Shoe --> shoes  : book--> books

Rabbit ---> Rabbits

Phone---> Phones

Road----> Roads

A few words have very irregular  forms in the plural :

one man--> two men
one woman --> two women
one person--> two people
one foot--> two feet
one loaf--> two loaves
one half--> two halves

A small set of words do not change form in the plural:

one mouse--> two mouse
one sheep---> two sheep
one piece of furniture --> many pieces of furniture


Words of Greek or Latin Origin which have retained their  original endings will  generally take the plural form associated with  the Language they are  drawn form :

One alumnus ---> two alumni
One syllabus --> two syllabi
one cactus---> two cacti