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Difficulty: MediumModal Auxiliaries and Expressing Modality

Complete the sentence below by filling in the blank with the correct modal verb phrase to express a past unfulfilled obligation.

Answer:Following the sudden failure of the primary cooling system, the senior technician stated that the operator 【should have】 switched on the emergency backup generator immediately.

Answer

should have (or ought to have)
The phrase 'should have' (or 'ought to have') combined with the past participle 'switched' correctly expresses that the operator had an obligation or recommendation in the past that was not carried out.

Step-by-Step Solution

1
Analyze the temporal context and meaning of the sentence
The event occurred in the past ('stated', 'switched') and describes an action that was required/advisable but was not carried out.
The sentence expresses criticism or regret about a past omission.
2
Identify the appropriate modal structure for past unfulfilled obligation
The structure required is modal auxiliary ('should' or 'ought to') + 'have' + past participle ('switched').
'Should have' or 'ought to have' is the standard grammatical construction in English for an action that ought to have happened in the past but did not.

Key Concept

Expressing Past Unfulfilled Obligation or Regret
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Topics

Modal Auxiliaries and Expressing Modality

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