Question

Write the tense and form of the underlined verb.

Answer 1

So the verb is "to use" and the tense is the past conditional. This is indicated by the words, "has been". Like, "I have used," or something similar.

The word "had" or "have" is almost always indicative of either the past perfect or past conditional.

The opposite of that is the past, or preterite tense, as in it definitely happened, as in, "I used it".

Hope that helped

Answer 2

had been used is  the past perfect tense  in a passive voice

in an active voice, it would be "had used"

she had written a letter to Rita (active voice)
A letter had been written to Rita (passive voice)

The arithmetic mean had been used frequently in the statistics (passive V.)
They had used frequently arithmetic mean in the statistics (active V.)

so the tense is past perfect in passive voice

hope I helped !!