When the first word of the sentence is a negative, the main verb comes before the subject, as in a direct question. Common negative expressions are: never, hardly, no sooner, scarcely etc.
Incorrect: Never I have seen such a sight.
Correct: Never have I seen such a sight.
Incorrect: Hardly I have had any rest for a week.
Correct: Hardly have I had any rest for a week.
Incorrect: No sooner I had gone to bed than someone knocked on the door.
Correct: No sooner had I gone to bed than someone knocked on the door.
Incorrect: No sooner she completed one project than she started working on the next.
Correct: No sooner did she complete one project than she started working on the next. OR No sooner had she completed one project than she started working on the next.
Incorrect: Scarcely I had stepped out when it started raining.
Correct: Scarcely had I stepped out when it started raining.
