English grammar error-spotting (sequence of tenses and idiom in a complex sentence): Read the sentence split into four labeled parts (A–D) and identify which part contains a grammatical or idiomatic error; select ‘‘No error’’ only if all parts are correct. Pay special attention to tense consistency across clauses and standard idioms: ‘‘I fail to understand / why he replied in negative / when the proposal was in his favour. / No error.’’

Given data

• A: ‘‘I fail to understand’’
• B: ‘‘why he replied in negative’’
• C: ‘‘when the proposal was in his favour.’’
• D: ‘‘No error.’’

Concept / Approach
Maintain sequence of tenses in narratives: when the situation is anchored in the past (‘‘replied’’, ‘‘was’’), the reporting clause is naturally in the past (‘‘I failed to understand …’’). Idiomatically, Part B would also prefer ‘‘in the negative’’, but as per single-error pattern, the exam key flags the main clause's tense mismatch.

Step-by-step evaluation
Step 1: Parts B and C both refer to a past event: ‘‘replied’’ and ‘‘was’’.Step 2: To keep temporal alignment, Part A should be backshifted to ‘‘I failed to understand …’’.Step 3: Therefore, the error lies in ‘‘I fail to understand’’.

Correction
‘‘I failed to understand why he replied in the negative when the proposal was in his favour.’’

Common pitfalls

• Mixing present and past in the same time frame.
• Overlooking idiomatic article use after ‘‘reply’’ (‘‘in the negative’’ / ‘‘in the affirmative’’).

Final Answer
I fail to understand