If I had studied harder, I _________ the exam.

• Third conditional এর নিয়মানুযায়ী - 
- অনেকসময় 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:
If I had studied harder, I would have passed the exam.

Complete sentence: If I had studied harder, I would have cleared the exam.

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