Spanish language Q&A Forum

The kwiz question is: Cristina no sabe todavía ________ va a celebrar su cumpleaños.

The most import possibilities are dónde and donde.

If i look in the lesson: Difference between dónde and donde (with and without a written accent)

i see: if you can substitute it for "in what location", you can use dónde.

This answer was wrong, it should be donde. I don-t understand that, it seems to contradict the lesson.

If the direct object of a verb is an animal, but not a pet or cherished animal, for example a tiger, is the personal a still indicated?

Busco a un tigre. or,

Busco un tigre.

By process of elimination, I selected estuvieron as being the correct answer; however, my preference would have been estaban. 

See the example given below:

Mario estaba de camarero en un restaurante.
Mario was working as a waiter in a restaurant.

Here is the question with the answer:

Mis hijos _estuvieron_ de vendedores en una tienda.
My children were working as shop assistants in a store.

My choice:
Mis hijos _estaban_ de vendedores en una tienda.

Would you explain the reason for the answer being the preterite and not the imperfect, please.

I would like to ask how they are related and if one can be used instead of the other one

I am very confused. In the above lesson it describes when to use poder in the preterite indefinido.

in this lesson there seems to be No specific moment in the past or where speaker is outside the time frame 

This lesson "Conjugate poder in the preterite tense in Spanish (El Pretérito Indefinido)" it describes when to use the preterite indfinido when referring to a specific moment in past  and time it happened is relevent OR referes to pastwhere speakersees themselves outside the time frame