If I had tried again –
I could solve the problem.
I could have solved the problem.
Correct
I could solved the problem.
I could have solve the problem.
Explanation:
Correct Answer is: I could have solved the problem.
Conditional sentence এ if যুক্ত Clause-টি Past Perfect হলে Main Clause-টি Perfect Conditional হয়।
অর্থাৎ Subject এর সাথে would have/could have/might have এবং Verb এর Past Participle Form হয়।
Passive এর বেলায় would have/could have/might have + been বসে।
এখানে যেহেতু বাক্যটি Active voice এ রয়েছে তাই পরের অংশে I could have solved the problem. বসবে।