Learn French: Pour éprouvante que soit l'expédition, nous partons demain. - However grueling the expedition may be, we are leaving tomorrow.

This is a formal structure for concession. The pattern pour + [adjective] + que means "however [adjective]" or "no matter how [adjective]". It emphasizes the adjective, showing that despite this extreme quality, the main action will still happen.

Why is the verb before the subject in soit l'expédition?

In the pour... que structure, you generally invert the verb and the subject when the subject is a noun. It follows the pattern: pour + adjective + que + verb + noun subject. If the subject were a pronoun, you would use standard order instead: pour éprouvante qu'elle soit.

Why is soit in the subjunctive mood here?

The expression pour... que always triggers the subjunctive. It sets up a hypothetical extreme or a concession, which is a classic use case for the subjunctive mood in French.

Does éprouvante have to agree with l'expédition even though they are separated?

Yes. Even though the adjective is moved to the front of the clause after pour, it is still the adjective describing l'expédition. Since the noun is feminine singular, the adjective takes the feminine singular form éprouvante.