Choose the most suitable option to replace the highlighted part of the sentence: No sooner had I reached the station when the train departed.

Correct — A. The trigger is No sooner. Sooner is a comparative, and a comparative is completed by than, not by when.

The first half is already right: after a fronted No sooner the auxiliary comes before the subject, had I reached. The second half keeps normal order in the simple past: than the train departed → option (a).

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

Why the others are wrong

• (b)
  then the train departed — Then is a time adverb, not the comparative word than. The two differ by one letter, and after sooner the comparison needs than.
• (c)
  when the train had departed — Keeps the wrong partner when, and turns departed into had departed. The later event, the train leaving, stays in the simple past.
• (d)
  than did the train depart — Than is right, but the inversion belongs to the No sooner clause, had I reached. The than-clause keeps normal order: than the train departed, not than did the train depart.

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

Fronting No sooner inverts its own clause: had I reached, not I had reached. Here the earlier event is past perfect (had reached) and the later one simple past (departed).

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.
• A fronted No sooner inverts its clause: No sooner had I reached.
• The than-clause keeps normal word order: than the train departed.

Common traps

• Writing when after No sooner by analogy with Hardly … when
• Carrying the inversion into the second clause: than did the train depart
• Reading then for than, since the two options differ by a single letter

No directly related past PYQ was found.