Can someone explain to me why these answers are incorrect? then explain the correct the answer?

Question 1. Juan buys flowers for us.

Juan compra flores para nosotros.  

Juan nos las compra.  

Answer 1: nos las

Explanation: Transitive verbs need a direct object to complete their meaning. The direct object of the verb to buy is "flores", this is a feminine and plural noun, so the direct object pronoun is "las" that matches gender and number with the object it replaces. The indirect object is "nosotros", which is a first plural person pronoun and the indirect object pronoun is "nos" because it has to match in number with the object it replaces. When the direct and indirect object pronouns appear in the same sentence, the indirect object pronoun is always written first, so in this sentence it is "nos las".

Question 2: She*

Answer 2: Ella reciclará la lata de refresco.

Translation 2: She will recycle the soda can.

Question 4: The students*

Answer 4: Los estudiantes tendrán una fiesta mañana.

Translation: The students will have a party tomorrow.