What does it mean when the verb ends with ~다 in past tense? When is this used?
This is called 해라체, it's one of the seven Korean speech levels, also known as the 'plain style', because it's formal but non-polite, like writing in the 3rd person. It's basically used in these situations:
- To younger or lower-rank people
- To close friends and relatives of the same age
- In impersonal writing (books, newspapers, magazines)
- In reported speech ("he said that...")
You can find a lot of resources online about it, but basically, when you use the plain style, you conjugate a verb in the past tense with 었다/았다/했다, depending on the verb stem. That's how 떠나다 becomes 떠났다.
Is it appropriate to use "당신" for you? My Korean friends said it is weird if I call them that. They said only couples or people arguing with each other will actually say "당신" to each other. And if you say "당심" instead of "너", why are you using "plain style."
You're right, but I think it's also used in advertisements and songs to a general, non-specific you. I'm not sure why they used 당신 instead of 너 here though, since it's in plain style. Maybe the context is a married couple arguing...
It's an important tense to know if you want to pass the standardized test of Korean language learning (TOPIK), starting from level 3 where you have to write an essay.