Hello Pan, 'su' as a possessive adjective means 'his' ,'her', 'your' , 'their' , 'one's or 'its' : su libro -> his/her/your/their book sus libros -> his/her/your/their books (' sus' used for the plural nouns) 'su' as possessive and pronoun means 'his' , 'her' , 'their', 'its' or 'one's ' as in: "Se alaba a un soldado por su valor, una mujer por su hermosura, a una casa por su situación y a los libros por su mérito " " A soldier is praised by his courage, a woman by her beauty, a house by its situation and books by their merit. "

原来你是中国人，呵呵~ su 在“他的\她的\您的一个东西的时候”用，sus在“他的\她的\您的许多东西的时候”用。这是我的理解，上面的例句是我自己编的，希望能让你理解。我也刚开始学西语，请多多指教，呵呵~

Both su and sus means "his" "her"or"yours",here "yours" is been used at a fomal situation. the difference is ,for example, when we say I'm his friend, we say "soy su amigo", while we say we are his friends, we say"somos sus amigos".The difference is what belongs to him is one thing or many things. I'm a beginner of spanish.Hope it could be helpful for you!

Su (Singular) ---> Su nombre es Natalia. Sus (Plural) -----> Sus nombres son Natalia y Alberto. Me gusta SU imagen de italki. ¿Son SUS gatitos?