To determine the correct word to fill in the blank, we need to understand the grammatical rule for the phrase "did nothing but". This phrase is followed by the base form (infinitive without "to") of the verb.

The structure of the sentence is: Subject + did nothing but + [base verb].

Let's analyze the sentence: "The boy did nothing but _____ throughout the lesson."

The verb in question is "yawn". Its base form is "yawn". Therefore, the correct choice is "yawn".

Step-by-Step Reasoning:

Step 1: Identify the key phrase in the sentence, which is "did nothing but".

Step 2: Recall the grammatical rule: After "do nothing but", we use the bare infinitive (the base form of the verb).

Step 3: Apply the rule. The base form of the verb is "yawn".

Step 4: Check the other options to confirm why they are incorrect:

• "yawning" is a gerund/participle, which is not used after this phrase.
• "to yawn" is the full infinitive (with "to"), which is also not used after this phrase.
• "yawns" is the third-person singular present tense form, which does not agree with the structure.

Final Answer: The correct word to fill in the blank is yawn.

The complete sentence is: The boy did nothing but yawn throughout the lesson.

Related Topic: Verb Forms after 'But'

This question tests your knowledge of which verb form to use after the word "but" in certain constructions. The rule is specific to phrases like "do nothing but", "cannot choose but", or "cannot help but". In these cases, "but" acts like a preposition meaning "except", and it is followed by the bare infinitive (the base verb).

Related Formulae/Rules

Rule: The structure [Subject + did/does/do + nothing but + Base Form of Verb] is used to indicate that the subject performed only that single action.

Formula: Subject + did/does/do + nothing but + V₁

Examples:

• She does nothing but study.
• They did nothing but complain.