Answers:
In Spanish (Castilian) the imperfect tense (indicative mode) is used to: 
1. talk about past habitual actions (adding used to in its different forms) 
2. describe what someone was doing when it was interrupted by someone (or something) else. In fact, is more a combination of the past tense and a continuous or repeating aspect. Frequently, this includes a sense of incompletion in the verb, but not always. (Is the equivalent to the Past Progresive Tense in english) 
 
In other words: 
It is specially used to describe what used to happen or what was happening. 
 
Knowing this, let’s begin with the answers related to the imperfect tense: 
 
Question 13 – Right answer: estudiaban 
 
 In this sentence we are talking about students, which are the third person in plural they. 
 
Now, the imperfect tense of the spanish verb estudiar (to study) with the third person in plural ellos (they) is estudiaban. 
 
Therefore the complete sentence is: 
 
Los alumnos estudiaban para el examen 
The students were studying for the exam 
 
 Question 14 – Right answer: volaban 
 
In this sentence we are talking about birds (pájaros or aves) which is a plural noun. 
 
Now, the imperfect tense of the spanish verb volar (to fly) with the third person in plural ellos (they) is volaban. 
 
Therefore the complete sentence is: 
 
Los pájaros volaban sobre los árboles. 
The birds were flying over the trees 
 
 Question 17 – Right answer: trabajaba 
 
This sentence is a good example of the usage of the imperfect tense to talk about past habitual actions with the addition of used to. 
Here we are talking about the first person in singular yo (I), and the imperfect tense of the spanish verb trabajar (to work) with this subject is trabajaba. 
 
Therefore the complete sentence is: 
 
Yo trabajaba los fines de semana. 
I used to work on weekends 
 
 Question 19 – Right answer: era 
 
In this sentence we are talking about the first person in singular yo (I). The imperfect tense of the spanish verb ser (to be) with this subject is era (was). 
 Therefore the complete sentence is: 
 
Yo era muy inteligente. 
I was very intelligent 
 
 Question 21 – Right answer: esperaba 
 
In this sentence Juanita is the third person in singular ella (she), this means the imperfect tense of the spanish verb esperar (to wait) with this subject is esperaba. 
 
Therefore the complete sentence is: 
 
 Juanita esperaba el autobús. 
Juanita was waiting for the bus 
 
Question 22 – Right answer: ibas 
 
This is a sentence written with the second person in singular tú (you) and the imperfect tense of the spanish verb ir (to go) with this subject is ibas. 
 
Therefore the complete sentence is: 
 
Tú ibas al cine 
You were going to the movie theater (cinema) 
 
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Now, if we are comparing the characteristics (superiority or inferiority) of two persons, animals, things or two groups; generally in Spanish we use the words más and menos, as follows: 
 
Subject + verb + más/menos + que… 
 
or 
 
Subject + verb + más/menos + adjective / adverb / noun + que… 
 
The following sentences are a good example of these comparisons: 
 
Question 15 – Right answer: menos 
 
In this sentence we are comparing the stature of two animals, the hippopotamus and the giraffe, where the first one results to be less tall than the second . This means in this case we have to use the following rule: 
 
Subject + verb (to be) + menos + alto (adjective: tall) + que… 
 
El hipopótamo es menos alto que la jirafa. 
The hippopotamus is less tall than the giraffe 
 
 Question 16 – Right answer: más 
 
In this sentence we are comparing the bravery of two animals, the lion and the donkey, where the first one results to be braver than the second. This means in this case we have to use the following rule: 
 
Subject + verb (to be) + más + valiente (adjective: brave) + que… 
 
 El león es más valiente que el burro. 
The lion is braver than the donkey. 
 
 Question 18 – Right answer: más 
 
In this sentence we are comparing the size of two animals, the hen and the chicken, where the first one results to be bigger than the second. This means in this case we have to use the following rule: 
 
Subject + verb (to be) + más + grande (adjective: big) + que… 
 
 La gallina es más grande que el pollito. 
The hen is bigger than the chick. 
 
 Question 20 – Right answer: menos 
 
In this sentence we are comparing the size of two animals, the goat and the cow, where the first one results to be smaller than the second. This means in this case we have to use the following rule: 
 
Subject + verb (to be) + menos + grande (adjective: big) + que… 
 
 El chivo es menos grande que la vaca. 
The goat is less big than the cow (or The goat is smaller than the cow)