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1. What would have been the scene, if Mr. David has witnessed this all today?

2. I serious think that the goals are unrealistic to achieve. But even it’s true that nothing is impossible.

3. Had you informed me earlier, I would have attended the seminar, it would have helped me to understand the basics in-depth.
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1. What would have been the scene, if Mr. David had witnessed this all today?
2. I seriously think that the goals are unrealistic to achieve. But then even it’s true that nothing is impossible.
3. Had you informed me earlier, I would have attended the seminar. It would have helped me to understand the basics in-depth. - seems ok to me
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Creativeguru1. What would have been the scene, if Mr. David has had witnessed this all day today?

2. I serious seriously think that the goals are unrealistic to achieve. But even Still, it’s true that nothing is impossible.

3. Had you informed me earlier, I would have attended the seminar . , it It would have helped me to understand the basics in-depth in depth.To me, the hyphenated form is an adjective, as "in-depth training."