Apart from knowing how to form the preterite - there are some words that can help us know when to use the preterite; these are called adverbios de tiempo (adverbs of time).
If we think about moments on a timeline, we can place moments and our adverbios de tiempo in the past, present, or future.
Here are some adverbios de tiempo that we use with the preterite. Can you think of any more?
esta mañana   this morning
ayer   yesterday
anoche   last night
anteayer   the day before yesterday
la semana pasada   last week
el lunes pasado   last Monday
el mes pasado   last month
el 13 de noviembre   November 13th
en 1998   in 1998
de repente   suddenly
hace [número] días, semanas, meses, años   [number] days, weeks, months, years ago
Tomamos las frases de Lucía in Lección 5.2:
Yo nací.
Yo caminé.
Estudié una carrera.
Fui de vacaciones.
Me compré un coche.
Me jubilé.
Añadimos unos detalles a ver qué nos dice sobre su vida:
Yo nací el 13 de noviembre de 1955
Yo caminé a los dos años.
Estudié una carrera en biología cuando tenía 22 años.
Fui de vacaciones
el año pasado a Italia.
Me compré un coche la semana pasada.
Me jubilé ayer, ¡y ahora estoy muy feliz!
OJO: When we say an age that we were in the past (like “when I was eight years old”) we have to use an imperfect structure. It’s OK that you haven’t studied that yet. In order to say “when I was ____ years old” we use “cuando tenía ____ años”.
OJO: We can contrast our use of the preterite and “acabar de + infinitivo” here. Remember that “acabar de + inf.” means someone “just completed something”, so it is a way to talk in the past as well.
Acabar de + infinitivo   El pretérito  
Acabamos de mudarnos y estamos muy felices.   Nos mudamos y estamos muy felices  
We just moved and we are very happy. (The move is very recent)   We moved and are very happy. (The move could be further in the past, we aren’t sure).  
Acabo de subir la foto de mi primo en mi red social.
Subí la foto de mi primo en mi red social.
I just uploaded the picture of my cousin on my social network. (I did it already, very recently).   I uploaded the photo of my cousin on my social network. (I did it already, but don’t specify when).