It's the past perfect continuous tense.
The verb tense "will have been" is future perfect continuous tense. It is formed by using the modal verb "will," the auxiliary verb "have," and the main verb in its past participle form followed by "been" and the present participle (-ing).
"must" is an example of a defective verb. It has only the present tense. To get the meaning of the past perfect tense of must, you have to say something like"had been required" or "had been obligated".
The future tense of "have been" is "will have been."
Past tense.
Am is the present tense, first person singular conjugation of be. The present perfect tense of be is have/has been.
Been is the past tense of be.
This is the present perfect tense. She's been = She has been
I/you/we/they have been. He/she/it has been.
The past tense is was or were.The past participle is been.
"Has been" is actually the present perfect tense.
Was is a past tense conjugation of be. The present perfect tense of be is have/has been.I/We/You/They have beenHe/She/It has been
It had been on