how can you tell when a verb is imperfect, perfect, ect.

The answer to the question is:

The Imperfect Preterite Form is used when the time at which the action is performed is not specified:

While the Perfect Preterite Form is used when specifying the time in which it is performed, either the time or the day.

Explanation:

In Spanish there are two ways in which a verb can be conjugated in Preterite Tense: the Imperfect Form and the Perfect Form, although at first glance it could be thought that they can be used interchangeably, the Imperfect Form serves to mention past tenses without mentioning the time in which they occurred, meanwhile the Perfect Form regularly specifies the time in which the events occurred, either within the complement of the sentence or in previous information.

corras is the only one that is informal but doesn't follow the suffix rule. correr is the verb. corre is the formal verb form. corra would be used as the formal is corre ended with an a.

your best bet would be corras, it's the only one that is informal but is still wrong