In this English grammar question, a sentence is given in active voice. Select the option that best expresses the same idea in passive voice. Sentence: "I could not find the book."

Introduction / Context:

This item uses a modal verb "could" in the negative form. The original sentence says that the speaker was not able to find a book. The problem asks you to choose the passive voice version that keeps the same sense of inability in the past.

Given Data / Assumptions:

- Original sentence: "I could not find the book."

- Subject in the active sentence: "I".

- Object: "the book".

- The modal verb is "could" with a negative, expressing past ability or possibility that did not exist.

- The passive sentence must convey that the book could not be found by the speaker.

Concept / Approach:

When forming passive voice with modal verbs such as "could", the pattern is "modal plus be plus past participle". The object "the book" becomes the subject in the passive sentence. The original subject appears after "by". The negative "not" stays after the modal verb and before "be". The verb "find" must change to the past participle "found" in the passive form.

Step-by-Step Solution:
Step 1: Identify the parts of the active sentence. Subject: I. Modal verb: could. Main verb: find. Object: the book. Step 2: Move "the book" to the subject position. Step 3: Use the passive modal pattern "could not be found". Step 4: Place "by me" after the verb phrase to show who was unable to find it. Step 5: Verify that the meaning still shows past inability to find the book.

Verification / Alternative check:

The sentence "The book could not be found by me." uses "could not be found", which is the correct passive form with a modal. It keeps the idea that the speaker was unable to locate the book and does not change the time reference. The roles of subject and object are reversed from the original, which is exactly what we expect in passive voice.

Why Other Options Are Wrong:

Some options use present tense auxiliaries such as "cannot", which change the time of the statement. Others have incorrect verb forms like "be find" instead of "be found". A few choices add unnecessary adverbs or time words like "now" that are not present in the original sentence. There are also options with unnatural word order and missing auxiliary verbs, which makes them ungrammatical. These problems mean they cannot be accepted as correct passive equivalents.

Common Pitfalls:

Many learners forget that modal passive forms require "be" plus a past participle and mistakenly write "could found" or "could be find". Another problem is mixing present and past modals, for example changing "could not" into "cannot". It is also common to overlook the correct placement of "not", which must follow the modal verb. Careful attention to the pattern "modal plus not plus be plus past participle" helps build correct sentences.

Final Answer:

The correct passive voice sentence is "The book could not be found by me."