First, kKorea hais surrounded by three seas, so there are many beautiful beaches are in Korea. First, Korea is surrounded by three seas, so there are many beautiful beaches Korea.

Korea is a proper noun and proper nouns should always be capitalized.

Many foreigners think , Korea is a peninsula, but because of North Korea, we can’t use paths toaccess other areas of the continent. Many foreigners think Korea is a peninsula, but because of North Korea we can’t access other areas of the continent.

That’s why we usually say Korea is an island ion the continent. That’s why we usually say Korea is an island on the continent.

Use the preposition "on the continent," not "in the continent."

NextSecond, Korea has a lot of mountains that more than 70 percent of land are mountains. Second, Korea has a lot of mountains that more than 70 percent of land are mountains.

You said "first" above, therefore here it is best to say "second."

So many Korean have their habit, hickings love to hike. So many Koreans love to hike.

Use the plural - Koreans. If it's singular, it's "a Korean." "Have their habit" doesn't make sense. We have hobbies. Here is it best to say "love to hike." Alternatively, you could say, "Many Koreans enjoy hiking as a hobby."

I usually go there with my sons in summer and swimming there. I usually go there with my sons in summer and swim there.