Whats the difference between the imperfect and the preterite? in the indicative category?

Question

for example: yo pensaba y yo pense or yo llegaba y yo llegue

can both be used the same?

Answer 1

Put simply, the imperfect tense is more like "I was doing something" or "used to do something" while the preterite means kind of like "I did something" and is more definite. The imperfect is used for things that were repeatedly done in the past and for like an unspecified, usually longer, period of time. While, the preterite is the opposite and used for things that happened once or are like in a concrete, set time. Hope this kind of helps! :)

Answer 2

The imperfect is used for descriptions of things which were in existence over a period of time, or for continuous actions. Eg. The night was dark and the moon was concealed behind the clouds (imperfect). I  was writing a letter home but I could not think of what to say (imperfect).  

The preterite is used for completed actions in the past.

Suddenly I sat up (preterite). Someone was knocking at the door (imperfect). I got up from my chair and opened it (preterite). Outside I saw a woman (preterite).