Bold answers:
Querido Fabián: Esta semana, mis padres (1)están en Bayamón, Puerto Rico. Mañana van a viajar en tren. (2)Los va a llevar a San Juan donde van a estar otra semana. Mi hermano y yo (3)estamos en casa de mis tíos Sandra y Ramón en el campo y estamos un poco (4)aburridos aquí. Yo tomo fotos todos los días, pero mi hermano no (5)las mira. Él está (6)enojado porque quiere regresar a la ciudad a ver a sus amigos. Hoy, está (7)haciendo calor y mañana va a hacer frío. Yo (8)estoy junto a la piscina descansando. ¡(9)Es un día hermoso! Tu amiga.
<h3>Translation:</h3>
Dear Fabián: This week, my parents are in Bayamón, Puerto Rico. Tomorrow they will travel by train. It is going to take them to San Juan where they will be another week. My brother and I are in the house of my aunt Sandra and my uncle Ramón in the country and we are a little bored here. I take pictures every day, but my brother does not look at them. He is angry because he wants to return to the city to see his friends. Today, it's getting hot and tomorrow it's going to be cold. I am by the pool, resting. It is a beautiful day! Your friend.

Both say “it’s their store” it just depends whether you are talking about one or more people. Su is usually he/she but it can mean they/it if you are being non-gender specific. Personally I would go for Es sus tienda but I’m not a native Spanish speaker so I could be wrong
2. Sí, sírvela; No, no la sirvas.
3. Sí, trae la silla; No, no traigas la silla.
4. Sí, lávalos; No, no los laves.
<span>It is subjunctive because it is an expression like you said the person is telling another that they WANT the packback
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