Learn Spanish (Latin America): Mirabas la rama cuando de repente apareció una serpiente. - You were looking at the branch when suddenly a snake appeared.

This is a classic example of an interrupted action! We use the imperfect tense for the background action that was already happening (mirabas, meaning 'you were looking'). We use the preterite tense for the sudden event that interrupts it (apareció, meaning 'it appeared').

Is de repente a clue for which past tense to use?

Yes! De repente means 'suddenly'. Because it describes an event that happens at a specific, sudden moment rather than a continuous background action, it is almost always a trigger for the preterite tense, just like apareció in this sentence.

Why does it say apareció una serpiente instead of una serpiente apareció?

In Spanish, it is very common to put the verb before the subject when introducing something new into the scene, especially with verbs of happening, arriving, or appearing. Saying apareció una serpiente sounds much more natural to a native speaker than putting the snake first.

English says 'looking at the branch'. Why is there no word for 'at' before la rama?

The Spanish verb mirar directly means 'to look at' or 'to watch'. The 'at' is already built into the verb, so you do not need to add an extra preposition like a or en before la rama.