Re-write the following sentences according to the instructions given after each. Make other changes that may be necessary, but do not change the meaning of each sentence.
Hardly had the teacher left the room, when all the children started making a noise. (Begin: No sooner ..................)
Topic: Transformation — 'no sooner'
Answer
No sooner had the teacher left the room than all the children started making a noise.
This converts between the two constructions that mean the same thing but take DIFFERENT partner words: HARDLY / SCARCELY ... WHEN NO SOONER ... THAN
So the only change required is WHEN becoming THAN. Everything else — the inversion ("had the teacher left") and the past perfect — is already in place, because both constructions use them.
Writing "No sooner had the teacher left... WHEN the children..." is the error the question is designed to catch: it keeps the partner word of the construction you were given rather than the one you were asked for.
Note also that the comma before "when" in the original should go, since "than" joins the two clauses directly.
The family of openers that force inversion: Hardly, Scarcely, No sooner, Never, Seldom, Rarely, Not only, Little.