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On the Independence Movement Day, hundreds of thousands of people from all over the country spontaneously gathered at Gwanghwamun and other places in Seoul to protect Korea's liberal democracy. I was there with my mom as well. Many young people who had been enlightened by the president's martial law also joined in, and the sight of patriotic elders welcoming them was very impressive.

On the Independence Movement Day, hundreds of thousands of people from all over the country spontaneously gathered at Gwanghwamun and other places in Seoul to protect Korea's liberal democracy. I was there with my mom as well. Many young people who had been enlightened by the president's martial law also joined in, and the sight of patriotic elders welcoming them was very impressive.
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Here are the corrections:

1. Original: On the Independence Movement Day, hundreds of thousands of people from all over the country spontaneously gathered at Gwanghwamun and other places in Seoul to protect Korea's liberal democracy.
   Corrected: On Independence Movement Day, hundreds of thousands of people from all over the country spontaneously gathered at Gwanghwamun and other places in Seoul to protect Korea's liberal democracy.
   Explanation: The article "the" is not needed before "Independence Movement Day" because it is a proper noun and a specific holiday.

2. Original: I was there with my mom as well.
   Corrected: (No change needed.)

3. Original: Many young people who had been enlightened by the president's martial law also joined in, and the sight of patriotic elders welcoming them was very impressive.
   Corrected: Many young people, who had been affected by the president's declaration of martial law, also joined in, and the sight of patriotic elders welcoming them was very impressive.
   Explanation:

   • "Enlightened" is not the correct word choice here, as "enlighten" means to educate or bring awareness, which does not fit the context of martial law. "Affected by" is a more accurate way to describe the impact of martial law on the young people.
   • "The president's martial law" is not a natural phrase. "The president's declaration of martial law" is more grammatically correct because martial law is something that is declared, not something possessed.

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Revised Text:

On Independence Movement Day, hundreds of thousands of people from all over the country spontaneously gathered at Gwanghwamun and other places in Seoul to protect Korea's liberal democracy. I was there with my mom as well. Many young people, who had been affected by the president's declaration of martial law, also joined in, and the sight of patriotic elders welcoming them was very impressive.