Learn Spanish (Latin America): Le preguntamos eso.

Le is an indirect object pronoun. With verbs of communication like preguntar (to ask), the person you are speaking to is the indirect object. The direct object (the thing being asked) is eso (that). So literally, the sentence translates to "We ask that to him."

Since le can mean "him", "her", or formal "you", how do we know it means "him" here?

Out of context, we don't! It could easily mean "We ask her that" or "We ask you that." If the context isn't already clear from the conversation, a Spanish speaker would clarify by adding a él, a ella, or a usted at the end of the sentence.

Could preguntamos also mean "we asked" in the past tense?

Yes. For regular -ar and -ir verbs, the nosotros (we) form is exactly the same in both the present tense and the preterite (simple past) tense. You have to rely on context or time words like ayer (yesterday) to know which one it is.

Why is "that" translated as eso instead of ese or esa?

Eso is the neuter (gender-neutral) form of "that". You use it when referring to an abstract idea, a situation, or a previous statement rather than a specific physical object with a masculine or feminine gender. Since "that" here refers to a general question or concept, we use eso.