Select the most appropriate option to fill in the blank.

If you listened carefully, you _______

The correct answer is 'would understand'. 

Key Points

•  By reading the above sentence we find that:
  • The given question is an example of the Second Conditional. 
  • The Structure of the given sentence is:
    • If+Subject+V2, Subject+would+V1.
  • ​Hence, Option 1) is the correct answer. 
     

Correct Sentence; If you listened carefully, you would understand. 

Additional Information

• Other Two Important Structures of Conditional Sentences:
  • Structure of 'First Conditional': 
    • If+Present Indefinite, Simple Future​.
    • Example: If he works hard, he will surely get the job.
  • ​Structure of 'Third Conditional':
    • If+Subject+had+V3, S+would+have+V3.
    • Example: If I had seen you, I would have stopped my car.

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• We tend to go for option 4) but it would be wrong. 
• Because the given sentence is in the past tense. Hence, it will be followed by a part helping verb.