ATMs in subway stations might not display English on the screen so you must go to an ATM in a bank.
Tipping is considered degrading in restaurants, so a nice bow and thank you will do.
South Korea is one of those countries with four seasons. Watch out for the summer monsoon season between late June and the middle of July. In Seoul, temperatures vary greatly between summer and winter. The highest is 38°C (102°F) which occurs between July and August, while the lowest is about 10°C (14°F).