We use the preterite to tell about an action that happened once and was completed.

We use the imperfect to tell about habitual actions in the past.

We also use the imperfect to give background details.

#1 To describe permanent characteristics:

Ella es linda / She´s pretty.

#2 To tell the date:

Hoy es lunes. / It´s Monday.

#1 To indicate location:

Estoy en la casa./ I´m in the house.

#2 To describe temporary characteristics:

Está mojado / He´s wet.

Saber: to have knowledge / to learn, to found out
Conocer: knew for some time / to learn, met for the first time.
Querer: wanted, had the intention to / tried to
No querer: didn´t want something or to do something / refused to do something
Poder: an ability to do something in the past. / a specific time when an ability to do something wasn´t the case.