• If + simple present + will/shall + base form of the verb = 1st conditional.
• If + simple past + would/could/might + base form of the verb = 2nd Conditional.
• If + past perfect +would/might have + past participle. = 3rd Conditional.

• সুতরাং, নিয়মানুযায়ী সঠিক বাক্যটি হচ্ছে -  If I had seen him, I might have told him the matter 
- এই বাক্যের if যুক্ত clause এ if +  past perfect (had + seen) এসেছে তাই এরপরের অংশে might have + past participle form of the verb(told) হয়েছে।