In the following sentence improvement question, choose the alternative that correctly completes the correlative structure with "No sooner" so that the sentence is grammatically correct: "No sooner did the teacher come into the class when we stood up."

Introduction / Context:
This question tests your knowledge of the fixed correlative structure "No sooner ... than ..." used to describe two actions that occur almost at the same time, with one immediately following the other. Many exam questions focus on this pair because learners sometimes replace "than" with an incorrect conjunction like "when."

Given Data / Assumptions:

• Original sentence: "No sooner did the teacher come into the class when we stood up."
• The structure begins with "No sooner did."
• The second clause describes what happened immediately after the teacher entered: "we stood up."
• The link between the two clauses currently uses "when," which may be incorrect in this pattern.

Concept / Approach:
The standard construction in English is "No sooner had or did something happen than something else happened." The pair "No sooner ... than ..." is fixed, much like "Hardly ... when" and "Scarcely ... when." Therefore, when you start a sentence with "No sooner did," you must follow the second clause with "than," not "when," "since," or "then."

Step-by-Step Solution:
Step 1: Recognise that "No sooner did the teacher come into the class" is the first part of the correlative pair. Step 2: Recall the correct continuation: "than we stood up." Step 3: Replace "when" with "than" to restore the proper structure. Step 4: The corrected sentence becomes: "No sooner did the teacher come into the class than we stood up." Step 5: Check that the sentence now reads smoothly and follows the pattern used in grammar books and exam keys.

Verification / Alternative check:
Compare with similar examples: "No sooner had I reached the station than the train left." If you insert "when" instead of "than," the sentence sounds incorrect. This confirms that "than" is the only acceptable conjunction in the correlative pair with "No sooner."

Why Other Options Are Wrong:

• since: "Since" generally introduces a reason or a time reference, not an immediate consecutive action in this special structure.
• then: "Then" is an adverb of time that can show sequence, but it is not the standard conjunction used in the "No sooner" construction.
• No improvement: Keeping "when" after "No sooner did" leaves the sentence ungrammatical in standard formal English.

Common Pitfalls:
Learners sometimes mix up the three similar structures: "No sooner ... than," "Hardly ... when," and "Scarcely ... when." They may incorrectly use "when" after "No sooner" by analogy with the other two patterns. To avoid this, memorise each pair exactly and practise with examples so that "No sooner ... than" becomes automatic when you see this type of inversion in exam questions.

Final Answer:
The correct improvement is than, giving the sentence: No sooner did the teacher come into the class than we stood up.