1. Preterite: This is used for completed actions in the past. It changes depending on the subject pronoun:
* Yo estuve (I was)
* Tú estuviste (You were)
* Él/Ella/Usted estuvo (He/She/You (formal) was)
* Nosotros estuvimos (We were)
* Vosotros estuvisteis (You all (informal) were)
* Ellos/Ellas/Ustedes estuvieron (They/You all (formal) were)
2. Imperfect: This is used to describe ongoing or habitual actions in the past. It also changes depending on the subject pronoun:
* Yo estaba (I was)
* Tú estabas (You were)
* Él/Ella/Usted estaba (He/She/You (formal) was)
* Nosotros estábamos (We were)
* Vosotros estabais (You all (informal) were)
* Ellos/Ellas/Ustedes estaban (They/You all (formal) were)
Choosing the correct tense:
* Preterite is used for single, completed actions. Example: "Yesterday, I was at the park."
* Imperfect is used for ongoing or habitual actions. Example: "I was living in Spain when I met my friend."
It's important to understand the nuances of both tenses to use "estar" correctly in the past.