This question tests your understanding of verb tenses and subject-verb agreement in English grammar. The goal is to choose the verb form that correctly completes the sentence, ensuring it logically and grammatically matches the context.

Let's analyze the options:

• knew: This is the simple past tense of "know".
• knows: This is the simple present tense, third-person singular form of "know".
• didn't know: This is the negative form of the simple past tense ("did not know").
• doesn't know: This is the negative form of the simple present tense, third-person singular ("does not know").

To solve this, you must determine two things:

Step 1: Identify the Subject
The subject of the clause is the person or thing performing the action. The correct verb form must agree with this subject in number (singular/plural) and person (first, second, third).

Step 2: Determine the Correct Tense
The tense (past, present, future) must fit the context of the sentence. Look for clues in the surrounding text that indicate when the action is happening.

Without the full sentence, we must assume the context requires a standard application of these rules. The most common correct choice in such exercises is often the one that maintains present tense and third-person singular agreement for a singular subject (e.g., he, she, it, a name). Therefore, knows or its negative form doesn't know are strong candidates, with the final choice depending on whether the sentence requires a positive or negative statement.

Related Topic: Tenses

Tenses indicate the time of an action or state of being. The three main tenses are Past, Present, and Future. Each can be simple, continuous, perfect, or perfect continuous. For this question, the focus is on the Simple Present and Simple Past tenses.

Simple Present Tense is used for habits, universal truths, and fixed arrangements. The third-person singular form typically adds an 's' to the base verb (e.g., he knows, she works).

Simple Past Tense is used for actions completed at a specific time in the past. It is often formed by adding '-ed' to regular verbs (e.g., walked, played), while irregular verbs have unique forms (e.g., knew, went).

Related Topic: Subject-Verb Agreement

This is a fundamental rule that a subject and its verb must be both singular or both plural.

• A singular subject ($he$) takes a singular verb ($knows$).
• A plural subject ($they$) takes a plural verb ($know$).

Formulae (Rules)

Present Tense, 3rd Person Singular:
Subject (He/She/It/Name) + Verb$+s$/$es$
Example: She knows the answer.

Present Tense Negative, 3rd Person Singular:
Subject + does + not + Base Verb
Example: He does not know the answer.

Past Tense:
Subject + Past Form of Verb
Example: I knew the answer.

Past Tense Negative:
Subject + did + not + Base Verb
Example: They did not know the answer.