EX: How simple it would have been had I been there.
The following are true:
1) "would" is a modal auxiliary verb.
2) "have" is an auxiliary verb.
3) "been" is an auxiliary verb.
4) "had" is an auxiliary verb.
5) "I" is a subject pronoun.
6) "been" functions as the main verb.
Here's the same sentence but re-ordered:
EX: Had I been there, it would have been how simple.
Note that, "had I" is another way of saying "If I had":
EX: If I had been there . . . . => Had I been there . . . .
Note also, "been . . . simple" is a copular structure; i.e., be simple, is simple, was simple, been simple.