@Alberto I think for a lot of verbs his rule works, but today for example, I heard tenia and tenia que many times when the English would have been the simple past tense (had & had to), so maybe for tener his rule doesn't work.

Yes Ric, He is right. In Spanish exists 2 past: Imperfect (Pretérito Imperfecto) and Preterite (Pretérito Perfecto Simple).

Imperfect is a past action that you don't know if it is finished. Preterite is a finished past action, for example:

Yo trabajé: before I was working but in this moment I am not working.

Yo trabajaba: in the past, I was working but you don't know if I am still working.

I hope it useful. (excuse my English)