Learn Russian: Куда мы будем отправлять эти документы? - Where will we be sending these documents?

This is the compound future tense, used for imperfective verbs (actions that are ongoing, repeated, or unfinished). It is formed by taking the future form of быть (to be) — which is будем for 'we' — and adding the dictionary form (infinitive) of the main verb (отправлять).

If the subject was 'I' instead of 'we', how would the future tense change?

Only the helper verb быть changes. For 'I', you would use буду. The main verb stays in the infinitive, so the phrase becomes я буду отправлять (I will be sending).

Why does the sentence use Куда instead of Где for 'Where'?

Где asks about a static location (Where are they right now?). Куда asks about direction or destination (Where to?). Because sending involves moving the documents to a destination, Russian requires куда.

What case is эти документы in this sentence?

It is in the accusative case because the documents are the direct object (the things being sent). For inanimate plural nouns like документы, the accusative form looks exactly the same as the basic dictionary (nominative) form. Эти is the matching plural word for 'these'.