I was thinking
Use this to say 'I was thinking about...' when the object of your thought is a noun. It's very direct.
Means 'I was left thinking' or 'It got me thinking.' Use this when an event or someone's words prompted your thoughts.
This version uses a different past tense (preterite). It implies you were thinking for a specific, completed amount of time. It's more advanced, but 'Estaba pensando' is safer for A2 learners.
I was thinking that we can order a pizza.
I was thinking about you all day.
After the class, I was left thinking about the problem.
What were you thinking about? You look very serious.
The phrase 'Estaba pensando' is the standard and most common form used across all of Latin America.
'Estaba pensando' is also the standard. The main difference is the use of 'vosotros' for 'you all,' so you might hear '¿En qué estabais pensando?' (What were you all thinking?).
While 'Estaba pensando' is used for 'I', you will hear 'vos' instead of 'tú' for 'you'. For example: '¿Vos qué estabas pensando?' (What were you thinking?).
Using 'pensé' instead of 'estaba pensando'. — 'Pensé' means 'I thought' (a single, completed action). 'Estaba pensando' describes the ongoing process of thinking in the past. Use 'Estaba pensando en una idea' (I was thinking about an idea), not 'Pensé en una idea' unless you mean you had a sudden thought.
Using the wrong preposition, like 'pensando sobre'. — In Spanish, you think 'en' something. Always say 'pensando en algo' (thinking about something), not 'pensando sobre algo'.
Literally translating 'I was thinking to do something'. — Don't say 'Estaba pensando a hacer algo'. The correct structures are 'Estaba pensando en hacer algo' or simply 'Estaba pensando hacer algo'.