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About halfway through writing this, I noticed that I should come back up here and talk about something to fill in a hole:
In English we are taught that it is incorrect to begin words with conjunctions (and, or, but, yet, ...). To use an example you probably remember from your elementary school...
The ball is blue. Sally likes balls.
...can be written as...
The ball is blue and Sally likes balls.
Maybe there's a comma there — I don't really remember to be honest — I'm not here to teach you English.
In Korean (and in Japanese, too, actually), it is perfectly acceptable to begin a sentence with a conjunction. So the following sentences would be equivalent and legal in this language:
The ball is blue. Sally likes balls.
The ball is blue and Sally likes balls.
The ball is blue. And Sally likes balls.
For the sake of emphasis, it is important to point out that that last one would not be right in English.
Also, in case you didn't know, the Korean word that you use when you want to start a sentence with the word "and" is 그리고. Furthermore, its important to point out that you don't use that word anywhere but at the start of a sentence.
Anyway, on with the lesson...
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In English, when we want to compare two contrasting things we use words like 'but' and 'yet'.
In Korean, when you want to compare two contrasting things, you either use the word 그런데 at the beginning of a sentence or you add -ㄴ(는/은)데(요) onto the verb.
Since I used a bunch of parentheses, I'll emphasize what that meant by writing out the different forms of it:
ㄴ데
ㄴ데요
는데
는데요
은데
은데요
Example:
The following sentences are equivalent in meaning.
제 친구는 미국사람 인데, 한국말 할 수 있어!
제 친구는 미국사람 이야. 그런데 한국말 할 수 있어!
+ Show Spoiler +
My friend is American, but he speaks Korean!
(PS to Korean people: the반말 form of 이다 is 이야, right?)
--edit: yes, yes it is. thanks.
Another example:
The following sentences are equivalent in meaning.
그걸 사고 싶은데, 지금은 돈이 없어. =(
그걸 사고 싶어. 그런데 지금은 돈이 없어. =(
+ Show Spoiler +
I want to buy it, but I don't have the money right now! =(
In addition to what I already said, you can also use these words 그런데 and 는데요 to introduce information that you are surprised about.
For example, in the first thing I gave up there about how my friend is American or whatever it was, I could have just as easily said "and" instead of "but" to make the sentence completely different... but I said "but" to emphasize on purpose that the thing I was talking about was not what you would normally expect.
On top of that, you can also use these words for when you don't want to repeat the question or speak in complete sentences.
Example:
-어떻게 오셨어요?
--김영수님을 만나러 왔는데요. 그는 있어요?
+ Show Spoiler +
- May I help you? (literally: For what reason have you come?)
-- I came to see 김영수. Is he in? (literally: Does he exist [here]?
-- I came to see 김영수. Is he in? (literally: Does he exist [here]?
Note that people who speak Spanish or Portuguese will be no stranger to the whole "is he there?" thing because they say it exactly the same way in their languages. (X esta?)
.. And that's all there is to know about that. (see what I did there?)