<span>The main difference between estar and ser is when they are used - estar is used for states which aren't permanent, meaning they can change. For example, a famous sentence from Mexican telenovelas: Estoy embarazada, which means - I am pregnant. It is not a state that will last forever. Having that in mind, here are the correct answers: 1. Nosotros estamos en la piscina.= We are at the pool. 2. Tu estás en la escuela. = You are at school. 3. Mi hermano y yo estamos en la cocina ahora. = My brother and I are in the kitchen at the moment. 4. Yo estoy en el comedor ahora. = I am in the dining room at the moment. 5. Hoy es Martes. = It's Tuesday today. 6. Son las tres de la tar.de. = It's three in the afternoon. (for hours, the verb ser is used). 7. Que hora es? = What time is it? 8. Estas tu contento hoy? = Are you happy today? 9. Hoy es Domingo. = It's Sunday today. 10. Los amigos son italianos. = My friends are Italian.</span>
Basically what the question is asking is to change the pronouns so that they make sense in the sentence. Either switch the pronouns to be past or present tense to best fit the sentence.
