<u>The Imperfect Preterite Form</u> is used when the time at which the action is performed is not specified:
<em>Yo </em><em><u>llegaba</u></em><em> a la casa de mi amigo y cenábamos juntos.</em>
While <u>the Perfect Preterite Form</u> is used when specifying the time in which it is performed, either the time or the day.
<em>Yo </em><u><em>llegué</em></u><em> a la casa de mi amigo a las 2:00 p.m.
</em>
<em>Ella </em><u><em>llegó</em></u><em> a la casa de su amigo el domingo.</em>
Explanation:
In Spanish there are two ways in which a verb can be conjugated in Preterite Tense: the <u>Imperfect Form</u> and the <u>Perfect Form</u>, although at first glance it could be thought that they can be used interchangeably, <u>the Imperfect Form serves to mention past tenses without mentioning the time in which they occurred</u>, meanwhile <u>the Perfect Form regularly specifies the time in which the events occurred, either within the complement of the sentence or in previous information</u>.
Imperfect and past tense verbs show the difference in the timing of past actions The past perfect tense relates to an action that was completed at a time relative to another timed event. For example, "By 2000 they had married." The imperfect tense, refers to an action that continued for some time.