Spanish uses a single possessive, su, for his, her, its, their — and for your when you say usted or ustedes. Like mi and tu, it only changes for number:

• su casa — his / her / their / your house
• sus llaves — his / her / their / your keys

Usually the context makes the owner clear. When it doesn't, put de + the person after the noun:

Owner	Form	Example
él	de él — his	la casa de él
ella	de ella — her	el piso de ella
ellos / ellas	de ellos / de ellas — their	el jardín de ellos
usted	de usted — your	el coche de usted

De él always stays as two words: only the article el, without an accent, merges — de + el → del.