Is it estar or ser? Is it the preterit or imperfect?
Questions to ask yourself when you are figuring it out:
- Is it measurable? Use the preterit.
- Is it unmeasurable? Use the imperfect.
I almost tried to explain it here, but I couldn´t explain it better then Gordon Smith-Durán from Lightspeed Spanish.
…or you can wait and buy my book when it comes out. 🙂 See this post.
Check out this YouTube video.