Pensé que era una broma, pero en realidad hablaba en serio.

I thought it was a joke, but actually he was serious.

Breakdown of Pensé que era una broma, pero en realidad hablaba en serio.

• yo: I
• ser: to be
• una: a
• pero: but

Questions & Answers about Pensé que era una broma, pero en realidad hablaba en serio.

Why is pensé in the preterite, but era and hablaba are in the imperfect?

This is a very common Spanish tense pattern.

• Pensé is preterite because it refers to a completed mental action: I thought
• Era and hablaba are imperfect because they describe how something seemed / was at that time

So the sentence is structured like this:

• Pensé = one completed moment of thinking
• que era una broma = what I believed the situation was
• pero en realidad hablaba en serio = what was actually going on

Spanish often uses:

• preterite for the main event
• imperfect for background, description, or ongoing context

Why is it era una broma and not fue una broma?

Because era focuses on how the speaker understood or interpreted the situation at the time.

• era una broma = it was a joke / I thought it was a joke
• fue una broma would sound more like identifying it as a completed fact or event

Here, the speaker is not saying what it truly was. They are saying what they believed it was. That is why era fits better.