(1) Right answer:
eran
This sentence stands for the imperfect tense. This tense is used to talk about past actions, conditions, or events that occurred regularly or frequently or that were in progress in a moment in the past. So in the past, they were young in progress. Since Anita y Jorge can be replaced by the subject pronoun for the third person plural ellos, then we need to choose the conjugation of the verb ser that matches the third person plural in the imperfect, which is eran. Finally:
Cuando Anita y Jorge eran jóvenes
(2) Right answer:
eran
This sentence continues from the previous one. Therefor, we continue to talk about the youth of Anita and Jorge. Therefore, what this sentence means is that they were good friend when they were children. That's what muy buenos amigos means very good friends. Lastly:
Cuando Anita y Jorge eran jóvenes
(3) Right answer:
iban
This sentence stands for the third person plural in the imperfect. Here we need to use the verb ir. Therefore, by conjugating this verb for the third person plural we have the word iban. Hence Ana and Jorge used to go to school together when they were children. Finally:
Ellos iban a la escuela juntos
(4) Right answer:
iban
This sentence also stands for the third person plural in the imperfect. Thus, we need to use the conjugation iban again. Therefore, they used to eat together when they were children. It is important to say that the verb comer is given in infinitive form. This is the most basic form of verbs in Spanish. Lastly:
Ellos iban a comer juntos
(5) Right answer:
era
This sentence stands for negation. Negation is an expression that means no and is used to contradict some or all of the meaning of a positive sentence. Negations using no is the most basic type of Spanish verb negation. To make a sentence negative, just place the word no before the conjugated verb. Since this sentence stands for the third person singular, the corresponding conjugation of the verb ser is era. Finally:
Anita no era muy buena alumna
(6) Right answer:
era
This sentence continues to talk about Anita, hence this sentence also stands for the third person singular, and we need to use the conjugation era again. Unlike the previous sentence, this one is positive. So we are saying here that she was a very good athlete. Finally:
Pero era muy buena atleta
(7) Right answer:
era
This sentence talks about Jorge, hence this sentence also stands for the third person singular, and we need to use the same conjugation era again. As you can see, this sentence is positive. So we are saying here that Jorge was an excellent student. Finally:
Jorge era un excelente alumno
(8) Right answer:
era
This sentence stands for negation. As we said above, by using basic negation, we just need to place the word no before the conjugated verb. Since we are talking about Jorge here, the correct conjugation of the verb ser for the third person singular in the imperfect is era. Finally:
Pero no era buen atleta