HARDLY, SCARCELY, BARELY, NO SOONER

http://www.grammaring.com/hardly-scarcely-barely-no-sooner

HARDLY / SCARCELY / BARELY … WHEN
NO SOONER … THAN

When a story is told in the past tense, the adverbials hardly, scarcely, barely and no sooner are often used to emphasise that one event quickly followed another. The verb describing the earlier event is usually in the past perfect tense. If hardly, scarcely, barely and no sooner are in the initial position, the subject and auxiliary are inverted:

Hardly had I arrived home when my phone rang. (I had hardly arrived home when my phone rang.)

Scarcely had she finished reading when she fell asleep. (She had scarcely finished reading when she fell asleep.)

Barely had they won the match when the coach had a heart attack. (They had barely won the match when the coach had a heart attack.)

No sooner had the company launched its new product than it went bankrupt. (The company had no sooner launched its new product than it went bankrupt.)

Note that hardly, scarcely and barely are followed by when, while no sooner is followed by than. (Sooner is the comparative form of soon.)