Fill in the blank with the correct inversion structure: No sooner had he entered the room ___ the fire alarm went off.

Correct — B. The trigger is No sooner. Sooner is the comparative of soon, and a comparative is completed by than.

The inversion is already in place: after a fronted No sooner the auxiliary comes before the subject, had he entered. The second clause then opens with than → option (b).

Pattern: No sooner + had + subject + past participle … than + simple past.

Why the others are wrong

• (a)
  when — When is the partner of hardly and scarcely: Hardly had he entered the room when the alarm went off. No sooner is a comparative and takes than.
• (c)
  but — But marks a contrast, and there is none here: the alarm simply follows his entry. It also leaves the comparative sooner without its than.
• (d)
  and then — And then narrates two events in sequence, as in He entered the room, and then the alarm went off. After a fronted No sooner, the comparison must close with than.

No sooner … than says that the second event followed the first at once. It is built on a comparative, sooner, so it closes with than, as in sooner than or faster than.

Put No sooner at the front of the sentence and the clause inverts: had he entered, not he had entered. Here the first clause is past perfect, had entered, and the second simple past, went off.

Keep the pairs apart: No sooner … than, but hardly … when and scarcely … when.

Key facts

• No sooner is completed by than, not by when.
• Hardly and scarcely are completed by when.
• When No sooner opens a sentence, the auxiliary comes before the subject: No sooner had he entered.
• Without fronting there is no inversion: He had no sooner entered the room than the alarm went off.

Common traps

• Writing when after No sooner by analogy with Hardly … when
• Reading the blank as a plain time link and picking and then, missing that No sooner has opened a comparison

No directly related past PYQ was found.