There are two ways to express simple past time actions and conditions in Spanish. One is the preterite,
Comí tacos. I ate tacos.
Besé a una chica. I kissed a girl.
and the other is the imperfect,
Comía tacos. I ate tacos.
Besaba a una chica. I kissed a girl.
and they are not interchangeable.
Sometimes the Spanish imperfect is compared to the English imperfect ("I was eating tacos.") or the used to paraphrase ("I used to kiss a girl."), but there are many cases where those don't mean the same thing as each other and even more where they don't mean the same thing as the imperfect in Spanish.
One of the differences is in indirect speech (see Does indirect speech in Spanish require changes in tense, mood, etc?) but there are other subtleties. How do you keep track of them and make them feel natural?