This question tests your understanding of modal verbs and perfect infinitives in English grammar. The sentence is a dialogue where one person is recounting what another said about a past action involving a screwdriver, and then questioning what the correct action should have been.

Let's break down the structure of the sentence to find the most suitable verb forms:

First Blank: "You say I ____________ a screwdriver."

This part reports what someone else said. The speaker is referring to a past possibility or a missed opportunity. The other person is suggesting that using a screwdriver was an option in the past. To express a past possibility, we use the structure: Modal + have + past participle (this is called the perfect infinitive).

From the options, "could have used" fits perfectly. It means "it was possible for me to use a screwdriver (but maybe I didn't)."

• "Shouldn't have used" implies a past mistake, which doesn't fit the neutral reporting tone.
• "Mustn't use" refers to a present/future prohibition.
• "Might not use" refers to a present/future possibility.

Second and Third Blank: "What _______ I __________?"

This is a question the speaker is asking themselves about the correct or best course of action in the past. They are seeking advice or reflecting on what the right thing to do was. To ask about a past obligation or the best action in hindsight, we use: should + have + past participle.

Therefore, "should" and "have used" are the correct choices. "What should I have used?" means "What was the correct tool for me to use in that past situation?"

• "could have used" would ask about possibility, not correctness.
• "be used" is a passive form and does not fit here.

Step-by-Step Solution:

Step 1: Analyze the first clause. It reports a past possibility. The correct form is a modal verb followed by "have" and the past participle. "Could have used" is the only option that expresses a past possibility.

Step 2: Analyze the question clause. It asks for the advisable past action. The correct form is "should" followed by "have" and the past participle. "should / have used" is the correct pair.

Final Answer: The most suitable verb forms to fill in the blanks are could have used, should, and have used.

The complete sentence is: "You say I could have used a screwdriver. What should I have used?"

Related Topics & Formulae

Modal Verbs for Past Actions:

We use modal + have + past participle (V3) to talk about the past.

• Could have + V3: To express past ability or possibility that was not acted upon.
  Example: I could have finished the work yesterday.
• Should have + V3: To give advice about the past or to talk about an expectation that something happened.
  Example: You should have called me.
• Would have + V3: Used in the result clause of a third conditional sentence.
  Example: If I had known, I would have helped.
• Might have / May have + V3: To express uncertainty about a past action.
  Example: She might have left already.
• Must have + V3: To make a logical deduction about a past event.
  Example: It must have been raining.