Watching a video from John’s wedding

[Maybo]

My friend wrote a sentence for me:

Watching a video from John’s wedding, the scene that struck Jane most was when the bride wore a pumpkin dress to swim.

Is that a grammatical sentence?

 

[tedmc]

Jane watched the video; the scene didn't watch.

 

[Barque]

the scene that struck Jane most

"Most" doesn't fit here.

Watching a video from John’s wedding, the scene that struck Jane most was when the bride wore a pumpkin dress to swim.

Your sentence sounds as if it's normal for the bride to take a swim at her wedding. It isn't. But assuming she did, as part of the festivities, try:

While watching a video of John's wedding, the scene that especially caught Jane's attention was when the bride wore a pumpkin dress in a swimming pool.
The scene that especially caught Jane's attention from the video of John's wedding was the bride jumping into a swimming pool wearing a pumpkin dress.

 

[Maybo]

Is it correct to start the sentence with “watching”? Should the sentence start with a pronoun?

 

[Barque]

Is it correct to start the sentence with “watching”?

It's possible. It depends on the sentence. It didn't sound right to me in your specific example.