You need to keep manner in Seoul.
South Korea is important about etiquette.
Korean citizens are generally kind and accepting of tourists. But in a culture that is far more conservative than America's, following etiquette is crucial. Respect is paramount, and Koreans may take offense at behavior that you wouldn't blink twice at witnessing at home. Tourists generally visit only South Korea; private American citizens aren't allowed into North Korea on their own. In any case, the same basic manners apply in either place. Pack fairly conservative clothing.
You don't need to cover every inch of skin, but don't expose your cleavage or wear mini skirts.
Always wear shoes that are easy to slip in and out of; whenever you enter a temple, you'll be expected to remove your shoes. Wear socks so you won't have to walk around barefoot. In general, pack clothing you'd be comfortable wearing to a dressy dinner out.(Reference USA Today)
Public Transportation
Traveling in Korea is easy and modern. Because of this, transportation options are often crowded, and pushing and cutting lines are common practices. Avoid arguments by holding firm to your space and always give your seat to elderly individuals.
This demonstrates respect.
Showing respect to anyone older than you is important when visiting Korea.(Reference kwintessential)
Gift Giving Etiquette
Do not sign a card in red ink.Use both hands when offering a gift.
Gifts are not opened when received.Gifts should be wrapped nicely.
The number 4 is considered unlucky, so gifts should not be given in multiples of 4.
Giving 7 of an item is considered lucky.(Reference kwintessential)