Use of preguntarse is straightforward if you're familiar with reflexive verbs. It can be literally translated as "to ask oneself," and has basically that same meaning.
- Me pregunto si es amor lo que siento o es solo un capricho. I wonder if it is love I am feeling or if it is only a whim.
- Nos preguntamos si este invierno volverá a nevar. We wonder if it will snow again this winter.
- Yo me preguntaba lo mismo. I wondered the same thing.
- ¿Qué es la vida buena?, se preguntaban los griegos. What is the good life? the Greeks wondered.
- Nunca se preguntaron como podía ser posible. They never wondered how it could be possible.
It is important to understand that "¿Dónde estarán las llaves?" does not (unless the context makes clear otherwise) mean "Where will my keys be?" There is a difference, however, between asking the direct question, "¿Dónde están las llaves?" (present tense, "Where are my keys?") and using the future tense as in "¿Dónde estarán las llaves?" In the latter case, the speaker isn't necessarily looking for an answer. Following are some other examples of what is sometimes called the suppositional future. In the examples below, two English translations are given. Either one (and possibly others) would be possible.
- ¿Quién irá a Fráncfort? I wonder who's going to Frankfurt. Who could be going to Frankfurt?
- ¿Qué querrá decir el autor en esta oración? I wonder what the author is saying in this sentence. What could the author be saying in this sentence?
- ¿Qué pensarán de nosotros en Japón? I wonder what the Japanese think about us. What could they be thinking about us in Japan?
- ¿Qué querría la policía con él? I wonder what the police wanted with him. What would the police have wanted with him?
- ¿Dónde estarían los secuestrados? I wonder where the hostages were. Where would the hostages have been?