Diane had been cooking sinigang for an hour by the time the bus arrived.

Sheila had studied already before going to sleep.

I had already walked before the school bus arrived.

Shows one action is completed before another action.

Used for a future action happening at a specific time.

They will have to review their lessons before taking the exam.

Mother will be packing our bags this time next year.

Mother will have been packing our bags for three hours by the time father gets home.

Used for an action that will be happening in the future before a specific length of time up to the moment when another action will happen.