Learn Icelandic: Deigið verður búið að hefast áður en baksturinn byrjar. - The dough will have risen before the baking begins.

In the construction verða búinn að, the word búinn acts as an adjective and must agree in gender and number with the subject. Since the subject deigið (the dough) is a neuter noun, búinn becomes the neuter búið.

How exactly does verður búið að capture the English future perfect tense?

Icelandic lacks a direct future perfect tense. Instead, it combines verða (used here for the future meaning of 'will be') with búinn að (finished doing) and an infinitive. By literally saying the dough will be finished rising, it perfectly translates the English concept of an action that 'will have' happened before a specific future event.

Why does the verb hefast end in -st?

This is the middle voice form. While the active verb hefa means 'to raise' or 'to lift' something else, adding the -st suffix gives it a reflexive meaning. Therefore, hefast means 'to rise' on its own, which is exactly what dough does.

Could I drop the en and just use áður here?