If the weather ______ improved, we could have gone for a walk.

• যদি Had/ If যুক্ত clause টি Past perfect tense হয় তাহলে পরের clauseটি তে subject এর পর would have/could have/might have বসে এবং এরপর verb এর Past participle form হয়।
Example:
If the weather had improved, we could have gone for a walk.
If I had tried, I would have passed.

Source:
1. Advanced Learner's English Grammar & Composition by Chowdhury & Hossain.
2. Cambridge Dictionary.