This sentence stands for the present progressive tense. This tense is used to talk about something that is happening now. To form the present progressive we use the following formula:
verb estar + present participle
Regular Present participle of ar verbs = verb stem + ending ando
Regular Present participle of er and ir verbs = verb stem + ending iendo
Finally:
Rebeca esta leyendo un libro
The imperfect tense is used to describe past habitual actions or to talk about what someone was doing when they were interrupted by something else. In general, the imperfect 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. To show a completed action in the past we use the preterite tense.
This sentence stands for the imperfect. So llevaban is the conjugation of the verb llevar for the third person singular in the imperfect. On the other hand, mis primos translates into my cousins, and party is fiesta. Finally, the sentence my cousins were getting along badly during the party is equivalent to mis primos se llevaban mal durante la fiesta in Spanish.
