How do you make the word ‘thief’ plural ?

Hint: A plural noun is one of the forms of noun which is used to refer to more than one entity. We use plural nouns when we refer to a quantity which is more than one of anything. Usually plural nouns are written the same as the word by simply adding -s, -es, -ies to the end. But there are some exceptional words where this thumb rule is not applicable. Example: Half-Halves.
Example: Child-Children.
Example: Hair- Hair.

Complete answer:
Let us see how we make the word ‘thief’ plural. The word thief means a person who steals. It is in its singular form that it refers to only one thief. More than one thief would be its plural form, that will be made by adding suffix -ves to the word ‘thief’ and eliminating the last letter ‘f’. Therefore, the plural of ‘thief’ becomes ‘thieves’. More such examples of plural are:
Half: Halves
Calf: Calves
Leaf: leaves
Words like roof, proof, brief etc are exceptions whose plurals are roofs, proofs, briefs

Note: A singular noun is a form of noun which represents only a single entity of the noun. Example: Pen, Cat, Key, House, Chair etc. A plural noun as mentioned in the hint represents more than one quantity . Example: Pens, Cats, Keys, Houses, Chairs etc. There are some words whose plural nouns are formed by not just adding -s,-es,-ies. The plural forms of some words are differently construed. Example: Man- Men
Example: Foot – Feet
A better understanding of plural nouns can be built only by thorough study of the topic.