Eight months in, there are a couple of things I have noticed about the way Koreans speak English, aside from having trouble with certain letters. These habits aren't necessarily incorrect, but they make the conversation sound very awkward, as if it doesn't belong in everyday conversation. The two biggest patterns are overusing certain words and overusing idioms.
The overusing of certain words is certainly understandable. I have regularly seen my colleagues in my office plugging Korean into an online translator and saying whatever comes out. The one word that immediately comes to mind is "envy." I probably hear this on a daily basis. "Oh, you can control that (specific) class. I am envious of you." "I cannot study at school, it is too difficult. You study hard. I envy you." "I envy your volleyball skills." "You live together but aren't married. I envy you." Most of the time, I think they mean "I'm jealous" or "I wish..." but "envy" comes off as a little extreme.
The overuse of idioms brings a more comical aspect to dialogue. I often wonder why instead of learning some more fundamental vocabulary or grammatical structures, the beginners speak in idioms. "Let's call (comes out as "carr") it a day"; "I have a frog in my throat"; "the child's behavior reared its ugly head"; "I'm in hot water"; "this book is as light as a feather." More often than not, the phrase does not come out quite right which just leads to confusion. Other times it sounds like something you might read in a newspaper or journal but rarely hear in everyday conversation.
I think I found the cause of pattern #2. Every morning, I get a ride to school with one of my co-teachers from last semester. During the drive to work, we always listen to an English morning radio program. Each day, they read a news clip and then define some of the sayings that were found in the article. A few minutes later, they quiz the listeners and you can call in to win a prize. This morning's phrase was "to hit the nail on the head."