Had I seen him, I ____________ him the news.

• Third conditional এর নিয়মানুযায়ী - 
- অনেক সময় conditional-এ subordinate conjunction if এর পরিবর্তে sentence এর শুরুতে Had ব্যবহার করা যায় এবং এর পর verb এর past participle form বসে।
- Conditional sentence- এ Had + Sub + V3 + Ext.
- এভাবে কোন Sentence শুরু হলে সেটিও Perfect conditional/ 3rd conditional হয়।
- তাই পরবর্তী clause এ would/could/might + have + V3 + extension ব্যবহৃত হয়।

Example:
Had I seen him, I would have given him the news.
Had I been a king, I would have helped the poor.

Source: Advanced Learner's English Grammar & Composition by Chowdhury & Hossain.