“has not seen” versus “has not been seen”

Question

In the sentence

The ocean, 95% of which has not (seen/been seen) before.

I know that the correct answer is "been seen". Why do I use been, the past tense of "to be"? Is there a better way to say this sentence?

Answer 1

This is a present perfect passive construction:

has been seen

Been is not the "past tense" of BE: it is its past/passive participle—let's abbreviate that PaPpl.

The preceding first verbform in the construction is has, a present-tense form of the perfect auxiliary HAVE. This auxiliary requires the following verb to take the PaPpl form, thus building a perfect construction.

Similarly, the BE here, in its PaPpl form been, is the passive auxiliary, which also requires a following PaPpl to build the passive construction: in this case, seen, the PaPpl of the final, lexical verb in the construction, SEE.

Answer 2

For your easy understanding here is an oversimplification. Decide first if the subject of the sentence is the doer or the receiver of the action. When the subject does his job, follow the normal tense pattern. If the subject is the receiver, take it for granted, BE is your main verb. Formulate tense with this idea in mind; don't search for meaning now. Irrespective of the tense used in the sentence, the main action verb would always be in past participle form( do- did- done)— the last one. The past participle form of the verb is almost an adjective in passive voice.

• The ocean, 95% of which has not (seen/been seen) before",

1.Ocean is the receiver. 2. Present perfect tense.3. (be- was- been) 4. has been 5. Seen ( always ) 6. Someone has not seen 95% of ocean before ( the active that had been)

It's a layman's guide, bereft of anything scholastic.