Imperfect Tense Question.

Question

Hello, like many people I struggle between choosing the imperfect or indefinido tense.

In this exercise, the sentence below was correct,

"pero ayer el guiso de mamá tenía carne,"

Please would you help me understand why the imperfect tense is correct, I had understood that the use of a time clause "ayer" would have made it indefinido.

Thanks

Answer

Hola Paul,
when the simple past tense is introduced to students at A2 level, there is an association between the use of this tense with certain time markers that trigger the simple past tense: "ayer", "el otro día", "el sábado pasado"... This is the specific usage taught at that level. As you advance in your Spanish you learn that you can actually use the imperfect with those same markers; it depends on what the speaker wants to convey and what sort of sentence it is. 
In this sentence from that exercise:
"pero ayer el guiso de mamá tenía carne",
we are using the imperfect tense of tener because what we convey is a description of that stew. We cannot see this description as something with a specific beginning and end. The time marker "ayer" doesn't automatically bring all verbs after it in the simple past. We are saying in this sentence that there was a meaty stew (mum's stew) and that was yesterday. The focus of "tenía" here is not to express something completed, it is more of a background/descriptive usage (even though that was happening yesterday).
Here are other examples with ayer and the imperfect:

Ayer había un accidente en la carretera hacia Sevilla.
Yesterday there was an accident on the road to Seville.

I could have also correctly used "hubo" here if I saw this accident as something with a beginning and end, but I chose "había" as a background/descriptive tense of haber.

Ayer no podía dormir porque tenía muchas cosas en la cabeza.
Yesterday I couldn't sleep because I had lots of things in my head.

You can see here that for the same reason as in the other example I chose to use the imperfect of those two verbs "podía and tenía", meaning that "I wasn't [being] able" and "I was having". But all of this was happening during yesterday. 
If I had said instead: Ayer no pude dormir, which is absolutely correct too, that simple past tense "pude" would indicate that I didn't manage to go to sleep, it shows a result, a completed action; while no podía dormir is not telling us if I managed in the end or not, there was some time during yesterday when I wasn't [being] able to sleep.
It is quite difficult to master this combination of time markers and tenses because of the limitations on the English translations, but I hope this helped a bit.
Saludos
Inma