If she _______________ how to calibrate the instrument, she _______________…

Correct answer: had known, could have

Why this is correct:

• This is a third conditional (an unreal past). It describes a past situation that did not happen and its possible past consequence.

• Form: If + past perfect (had + past participle), result = modal perfect (could/would/might + have + past participle).

• Example: If she had known how to calibrate the instrument, she could have done the experiment.

Why the other choices are wrong:

• knows, will have: Uses present and future-perfect forms, so it does not express a counterfactual past; it suggests a present/future situation instead.

• knew, had: Uses simple past and past simple, which does not convey an unreal past hypothetical and results in a grammatically odd or incorrect meaning here.

• should have known, would have: "Should have known" implies obligation, criticism, or regret rather than a neutral hypothetical condition. That changes the meaning from a simple counterfactual to blame or expectation.

Key tip: For unreal past conditions, use "had + past participle" in the if-clause and "could/would/might + have + past participle" in the result clause.