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Hva er forsjkellen mellom "Ja" og "Jo" ?

Det er et godt spørsmål. Jeg kan gi deg noen eksempler først, så ser vi om det er mulig å trekke noen generelle konklusjoner etterpå.

Since you´re on an elementary level, I think I´ll explain this in English, as it might get a bit advanced.
Good question. I´ll give you some examples first, and then we can construct some general guidelines for when to use "Ja", and when to use "Jo".

"Liker du kaffe?" ("Do you like coffee?")
- JA, det gjør jeg. ("Yes I do")

In this example "Ja" will always be correct.

"Liker du ikke kaffe?" ("Don´t you like coffee")
- JO, jeg synes det er kjempegodt. ("Yes, I like it very much")
or
- JO, men ikke så veldig. ("Yes, but not that much")

In this example, "Jo" will always be correct for both of the given answers.
This way of asking questions, with a negative, is very common in Norwegian, and when someone imposes a negative statement with a question, you disagree with "Jo".

There is no equivalent for "No". The opposite answer of "Ja/Jo" will always be "Nei".

Rule:
Positive question - agree with "JA", disagree with "NEI"
Negative question - agree with "NEI", disagree with "JO"

"JODA"/"JO, DA"
You should note that in colloquial norwegian, "JODA" is often used instead of "JO". It basically has the same meaning, but possibly a bit less enthusiastic. "Joda, jeg liker kaffe, men synes ikke det er kjempegodt" ("Yes I like coffee, but I don´t love it")