The correct answer is "would have". This is an inverted third conditional sentence. Had I known means If I had known, then I would have attended.

The correct answer is "had". Inversion with no sooner requires the past perfect and subject-verb inversion. No sooner had he arrived than it started raining.

The correct answer is "was". Inversion with not only requires the auxiliary verb before the subject. Not only was he late, but he also forgot his notes.

The correct answer is "have I seen". Inversion with rarely at the beginning requires the auxiliary have before the subject I.

The correct answer is "that". The structure It was not until that emphasizes the time when something happened.

The correct answer is "was". Inversion with so difficult requires the verb to be before the subject. So difficult was the exam that...

The correct answer is "did". Inversion with only after requires the auxiliary did before the subject she.

The correct answer is "had". Inversion with hardly requires the past perfect. Hardly had I arrived when the phone rang.

The correct answer is "did". Inversion with not until requires the auxiliary did before the subject he.

The correct answer is "did". Inversion with little at the beginning requires the auxiliary did before the subject they.

The correct answer is "would have". This is an inverted third conditional sentence. Had she studied harder means If she had studied harder, then she would have passed.

The correct answer is "have". Inversion with never requires the auxiliary have before the subject I.

The correct answer is "did". Inversion with so quickly requires the auxiliary did before the subject time.

The correct answer is "did". Inversion with only then requires the auxiliary did before the subject we.

The correct answer is "would have". This is an inverted third conditional sentence. Had it not been means If it had not been, then I would have failed.

The correct answer is "had". Inversion with scarcely requires the past perfect. Scarcely had the meeting started when the fire alarm rang.

The correct answer is "is". Inversion with not only requires the verb to be before the subject. Not only is she talented, but she is also hardworking.

The correct answer is "can". Inversion with only by requires the modal can before the subject you.

The correct answer is "did". Inversion with at no time requires the auxiliary did before the subject he.

The correct answer is "should". Inversion with under no circumstances requires the modal should before the subject this door.