Preterite vs Imperfect
Unlike the English language where there is only one past tense, Spanish has two different past tenses; the preterite and the imperfect. The verbs can be formed into the imperfect and preterite tenses according to the meaning. The regular verbs which end in –ar can be conjugated in either tense by using some particular endings.

Preterite
The preterite tense is used generally for already completed tasks or actions in the past tense. It also denotes that the action which was completed in the past had a beginning and an end. For example:

Dora spoke from two until eight.
Dora hablo’ de la dos hasta las ocho.
The beginning and the end of the action may or may not be stated clearly. For example:
Dora spoke for one hour.
Dora hablo’ una hora.

It refers to actions which might have happened once only. In simpler words, we can say this tense tells specifically when the task or action happened.
In typical cases, when reference is made to the interruption of an ongoing action, both tenses can be used. The preterite tense will be used for the action which interrupted the ongoing action; whereas the imperfect tense will be used for the action which was interrupted by another.
To use the preterite tense, the regular verbs with –ar endings are dropped and one of the following are added: é, aste, ó, amos, asteis, aron.

Imperfect
The imperfect tense is used for a task or action which was not considered finished or completed in the past. It also denotes that a particular action taking place in the past did not have a particular beginning or end. For example:

Dora used to speak in English.
Dora hablaban en ingles.

It shows the general time when the past action took place. This tense denotes an action which had been done more than once in the past. The actions which were done with regular frequency. For example:
Dora used to wake up every day at 6:00 a.m.
It expresses what was happening in the past at a particular time, for example:
The moon was rising; the sun was shining.
To use the imperfect tense, the regular verbs with –ar endings are dropped and one of the following are added: abas, aba, ábamos, abais, aban.

Summary:

1.The preterite tense is used to show an action which was completed in the past. It may or may not have a beginning or end. The imperfect tense is used to show actions in the past which were not considered finished.
2.The preterite shows the specific time when some event took place; the imperfect shows the general time when some event took place in the past.
3.The preterite tense is used to denote an action which is typically done once; the imperfect is used to denote an action which has been done frequently in the past.
4.In an ongoing action, when one action is interrupted by another, the action which was interrupted is denoted by the imperfect tense; the action which interrupted is denoted by the preterite tense.

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