Their is already plural, the possessive form of the third person plural (they, them). The word "theirs" is a predicate adjective form (such as mine, ours, yours, and hers).
The plural for "his" or "her" is " "their" ! Example sentence: His home was sold and became theirs.
The possessive adjective "its" would have the plural "their" or "theirs". Neither uses an apostrophe. Example : "The cat ate its food. The dogs ate their food." Example: "The car was theirs."
Theirs. It is a singular pronoun so has no plural possessive.
Both "they" and "theirs" are third person plural. The pronoun "they" is a personal pronoun, which functions as the subject of a sentence or a clause. The corresponding third person, objective form is "them". The pronoun "theirs" is a possessive pronoun, which takes the place of a noun belonging to two or more people or things.
The possessive pronoun 'theirs' is the pluralform, which takes the place of a noun belonging to a plural noun or two or more nouns.The singular possessive pronouns are mine, yours, his, hers, its.Examples:The Longs live on my street. The house on the corner is theirs.Jack and Jill live on my street. The house on the corner is theirs.
Theirs
The plural form for the pronoun 'she' is they (subjective) and them (objective); the plural possessive form is 'theirs'. A possessive pronoun takes the place of the noun that belongs to someone or something. Example sentence:The Collins sisters bought a house on my street. The house with the moving van must be theirs.
Subject (before a noun): (singular) my, your, his (plural) our, your, their Object (after a noun): (singluar) mine, yours, his, hers (plural) ours, yours, theirs. "Its" can be used in both subjective senses, but not objective.
The word its is the third person singular possessive pronoun.The word their is the third person plural possessive pronoun.There is no objective form of its, but the objective form of their is theirs.
The word THEY is listed as a plural pronoun, possessive their or theirs, objective them.See Related Link for details.
Theirs is the plural possessive pronoun for 'he'. For example:Mack and Mike bought a new boat, that one must be theirs.Their is the plural adjective for 'he'. For example:Mack and Mike bought a new boat, that must be their boat.
The pronoun 'his' is a possessive pronoun and a possessive adjective.The corresponding plural possessive pronoun is theirs.The corresponding plural possessive adjective is their.Examples:Jack got a new bicycle. The red one is his. (possessive pronoun)The twins got new bicycles. The red ones are theirs. (possessive adjective.Jack rides his bicycle to school. (possessive pronoun)The twins ride their bicycles to school. (possessive adjectives)