The main difference is that you can use the Second Conditional to describe a result that can happen although it is unlikely that it will, while you use the Third Conditional to describe a situation that could have happened in the past had a condition been met.
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Answer:
The main difference is that you can use the Second Conditional to describe a result that can happen although it is unlikely that it will, while you use the Third Conditional to describe a situation that could have happened in the past had a condition been met.