If they had known us, they …

Analyze the question

The sentence begins with "If they had known us..." This is a conditional sentence, and we need to choose the correct main clause to complete it.

Explain the concept: Third Conditional

When the "if" clause uses the past perfect (had + past participle), we are talking about an unreal situation in the past. The main clause must then use:

> would/could/might + have + past participle

Structure: If + past perfect, would have + past participle

Example: If she had studied, she would have passed.

Solution step by step

1. The "if" clause is "had known", which is past perfect, so this is the third conditional.
2. The main clause must be "would not have + past participle".
3. Check the options: • A. "will not treat": wrong, because it is simple future and does not match the past perfect. • B. "would not treat": wrong, because this pairs with the second conditional (If they knew us, they would not treat us...). • C. "will not have treated": wrong, because "will" cannot be used in the third conditional. • D. "would not have treated": correct, because it follows the pattern would + have + past participle.

Answer: The complete sentence is: If they had known us, they would not have treated us like that.

Exam tip

Match the tenses in conditional sentences:

• Present/likely: If + present, will
• Unreal present: If + past, would
• Unreal past: If + past perfect, would have

If you see "had + verb" after "if", look for "would have" in the options.