In the following question, a sentence is divided into four parts. One part may contain an error. Identify the part that has the error: No sooner had he finished (1) / his morning walk (2) / when it began to rain. (3) / No Error (4).

Introduction / Context:
This error spotting question examines your knowledge of a common correlative structure in English: no sooner ... than. Many learners confuse the second part and use when instead of than, which exam setters deliberately test. The sentence describes two events in quick succession, and you must recognise the correct conjunction that pairs with no sooner in formal English.

Given Data / Assumptions:

• Sentence parts: (1) No sooner had he finished; (2) his morning walk; (3) when it began to rain; (4) No Error.
• The intended meaning is that it began to rain immediately after he finished his walk.
• The standard pattern is No sooner had X happened than Y happened.
• We must select the part that violates this standard pattern.

Concept / Approach:
In English, certain expressions are used in pairs, known as correlative conjunctions. For very immediate succession of events we use no sooner ... than, hardly or scarcely ... when, and barely ... when. The combination no sooner ... when is incorrect in formal standard English. In the given sentence, the first part correctly uses no sooner with an inverted past perfect (had he finished), but the third part incorrectly uses when instead of than. This is the error that must be detected.

Step-by-Step Solution:
Step 1: Read the first part: No sooner had he finished. This follows the correct pattern where no sooner is followed by an auxiliary had and subject he, forming an inverted past perfect that is common in such structures. Step 2: Read the second part: his morning walk. This simply completes the first clause and is grammatically correct. Step 3: Examine the third part: when it began to rain. Here we must check the conjunction that joins the two clauses. Step 4: Recall the correct correlative pair: no sooner ... than. When we begin a sentence with no sooner, we must complete it with than before the second clause. Step 5: Therefore, the sentence should read: No sooner had he finished his morning walk than it began to rain. Step 6: Since when is wrongly used instead of than, the error lies in part 3.

Verification / Alternative check:
Compare with the related structure hardly had he finished his morning walk when it began to rain. In that pattern, when is correct. For no sooner, however, the matching word is than. Both structures express almost the same idea but use different conjunctions. If you substitute than for when in the original sentence, it immediately sounds more natural in formal written English. This shows that part 3 must be changed and is therefore the incorrect part.

Why Other Options Are Wrong:
Part 1 is correctly formed with no sooner followed by had and the subject; this inverted form is standard when no sooner begins a clause.
Part 2, his morning walk, is a simple and correct noun phrase that completes the object of finished.

Common Pitfalls:
Because hardly, scarcely and barely pair with when, many learners subconsciously copy the same structure with no sooner and produce no sooner ... when. Examiners exploit this overlap to set traps. A useful memory aid is: no sooner than and hardly or scarcely when. By memorising these as fixed pairs, you can quickly spot errors of this type in competitive exam questions and avoid confusing the conjunctions.

Final Answer:
The error is in part 3, where when should be replaced by than to form the correct structure: No sooner had he finished his morning walk than it began to rain.