1. A man is skiing down the slope of a mountain. Trees and the mountain are covered with snow. There are some air mails tied with colored string on the right side.

2. There are numerous people at the beach. They are having a good time with their close friends or family members. We can see parasols at the beach. It would be great to enjoy swimming in the sea.

3. It is 3:00 pm in Korea. What time is it in New Zealand? We have to add 3 hours to the time in Korea. So, in New zealand, it is 6:00 pm now.
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Would you check the passagea above?
If there are any errors, correct them, please.

Your sentences are correct. :-)

However, I don't know what "air mails" are. I've asked a Korean immigrant friend about them and if there's a better English word for them. I'll post again when I receive a response from her.

My friend didn't know what "air mails" are, either.

Please explain what "air mails" are and we'll try to help you find a better word or words for this sentence.

In sentence 1, are "air mails" the same things as kites?

http://images.google.com/images?hl=en&q=kites&gbv=2

In sentence 3, please delete the comma after "So" -- if "so" is used as a coordinating conjunction, a comma is needed BEFORE it; if there is a dependent clause after "so" then you'd use a comma after "so" and at the end of the clause, usually just before the main subject.

1. A man is skiing down the slope of a mountain. Trees and the mountain are covered with snow. There are some parcels tied with colored string on the right side.

2. There are numerous people at the beach. They are having a good time with their close friends or family members. We can see umbrellas at the beach. It would be great to enjoy swimming in the sea.
3. It is 3:00 pm in Korea. What time is it in New Zealand? We have to add 3 hours to the time in Korea. So, in New Zealand, it is 6:00 pm now.