Breakdown of D'où vient ce renfort ? Il sera venu du nord.
être
to be
il
he
ce
this (before a noun)
de
of / from / any (in negative sentences) / about (concerning)
Questions & Answers about D'où vient ce renfort ? Il sera venu du nord.
Why is sera venu translated as "must have come" instead of "will have come"?
This is the epistemic future. In French, the future perfect (le futur antérieur) is often used to express a strong probability or an assumption about a completed past action. Here, the speaker is making a logical deduction about where the troops originated right now, rather than making a prediction about the future.
Why does sera venu use être (sera) instead of avoir (aura)?
Because the main verb is venir (to come). Just like in the passé composé where you say il est venu, verbs of motion and state that take être as their auxiliary verb also take être in the futur antérieur. You simply put être in the simple future tense (il sera) and add the past participle (venu).
Is renfort a singular or plural concept here?
Why is it du nord and not just de nord?