Answers:
1. Juán es muy atlético.
2. Mi hermana es baja.
3. Yo soy inteligente.
4. Los libros no son fáciles.
5. Tú eres muy guapo.
Step-by-step explanation:
1. Instead of Juán, you can view it as saying él. According to the image (the one I provided below), you'd use es in this case.
2. Instead of "mi hermana," you can view it as saying ella. According to the image, you'd use es.
3. According to the image, when you see yo, you use soy.
4. 'Los libros' is talking about multiple things, like the word ellos. Knowing this, you'd use the word son.
5. According to the image, when you see tú, you use eres.
Just memorize the conjugations of SER and you'll be set!