Final answer:
In Spanish, to indicate possession or what belongs to someone, the verb 'ser' is used because it refers to permanent characteristics, which includes ownership. For example, 'El libro es tuyo.' indicates that the book is yours using the possessive pronoun 'tuyo' alongside the verb 'ser'.
Step-by-step explanation:
To indicate what belongs to someone in Spanish, you use the verb ser, because it relates to a characteristic or description that is considered permanent or inherent to the person or thing. For example, if you want to say 'The book is yours,' you would say, 'El libro es tuyo.' Here, 'es' is a conjugation of ser, and 'tuyo' is a possessive pronoun. Another example is 'The house is theirs.' which in Spanish is 'La casa es suya,' where 'suya' indicates that the house belongs to them.
It is important not to confuse the permanent state implied by ser with the temporary or variable state indicated by estar. Estar is used for conditions or states that are likely to change, like emotions or locations.