There is an API page for every Core SE Java class on the web which can be found easily by searching "java string doc", probably the first result of that search. Substitute 'string' with any other class name for that class' API page. You must know about this documentation but perhaps it has a different name in your language or classroom. If you don't know about these API pages, you might consider another book/class/instructor/institution for learning Java.
As for writing code to be easily tested, I modified your main() method as below to test your program:
public static void main(String[] args)
{
// set setning to a test value and call the method(s) to test
String[] setning = { "This is a sentence of English words." };
skrivHeleSetningen(setning);
// the same can be repeated as needed:
String[] setning2 = { "This is another sentence of a different"
+ " number of words." };
skrivHeleSetningen(setning2);
} // end method main()
My approach above changed your code a bit more than I normally would, but since your variable names were foreign to me anyway, I was more comfortable changing them to ones I understood.