Learn Spanish (Latin America): Tus camisas son bonitas.

In Spanish, possessive adjectives like "your" have to match the number of things being owned, not the number of owners. Because camisas (shirts) is plural, you have to use the plural form Tus. If you were talking about just one shirt, it would be Tu camisa.

Could I say Sus camisas instead of Tus camisas?

Yes, but it changes the tone. You use Tus when talking to a friend, family member, or peer (the informal tú). If you are speaking respectfully to someone you don't know well, a boss, or an elder (the formal usted), you would use Sus. Keep in mind that Sus camisas can also mean "his", "her", or "their" shirts!

Why is the adjective bonitas and not bonitos?

Adjectives describe nouns, and they must match the noun's gender (masculine or feminine) and number (singular or plural). Because camisas is a feminine, plural noun, the adjective must also be feminine and plural: bonitas.

Why do we use son here instead of están?