which one is most commonly used? Do they have slightly different meanings in different context?
They don't have slightly different meanings, they have completely different meanings.
"Du" is singular you
"Ni" is plural you
Some people might also tell you that "ni" could be used for formal singular you, but this not entirely true. I don't think it's very controversial in Finnish Swedish, but in Sweden many people, especially older people, might get offended if you try to use it as a formal you. Traditionally it was used as a marker for status and inequality, and this is something modern Swedes are trying to move away from. To show that everybody is equal, we prefer to use "du" in all cases where the word "you" is singular. Therefore I recommend to refrain from using "ni" as a formal singular you.
Aaah okay thanks! I didn't realize that. In Dutch (my mother language) there's also a difference in plural and singular 'you', so it won't be hard to remember :P Our formal 'you' is the same in plural and singular, though :P
Well, what I was trying to say is that Swedish doesn't have a formal you, neither in singular nor in plural.
Shouldn't the app let the user know that this was a plural "you" so no one gets confused? I know it's multiple choice, but I've already done this on typing answers where I couldn't turn up my sound.
In Esperanto, I recall someone said that ni means we and vi means you, isn't it?
You're right. In Esperanto it's the other way round, but the Esperanto word comes also from Italian voi=you and noi= we. You see Esperanto is a multilingual language which is worthy to learn.
dig is the object form. In other words: dig is to du as "us" is to "we".
what is the point of accent or Umlaut above the "ater" ?? "ä" is it necessary?
It isn't an accent or an umlaut. In Swedish, a, ä, and å are totally different letters and represent different sounds. It's like m and n. You can't exchange one for the other just because they look a little similar.
If you're on a computer, you should see a row of circles underneath the text box. You can click on these to get the letters. Otherwise, you can hold down the Alt key and type in the number codes for each letter. They are: ä = alt 132 å = alt 134 ö = alt 148 á = alt 160
Hope that helps!
I never learned that you could use ni or du. it was never in any of my lessons