You genuinely don't need to speak or read any Korean to survive here.
Subway: English
Banks: Most banks have english branches
Post office: Self servive machines in English
Supermarkets: Just look at the screen, or use a Homeplus, which is a Tesco and has English self service tills
Bars: Most bartenders speak, or understand English
Coffe shops: drinks are the same names.
restaurants: some are Korean completely. Others have English menus
McDonalds: Have an English language delivery website.
Hairdressers: Take a picture and point
Phone: Most have an English branch so you can sign up for your phone, internet, and cable.
Social life is easy. Almost everyone in Itaewon speaks English, so that will cover all drinking, eating, and
Hongdae is a student area so has a lot of English speakers.
Just learning a little Korean does go a long way, and learning the alphabet will make everything a lot easier.