Can "sak" also be used to mean "thing" in Norwegian, like it can in Swedish? Or is it a false friend between the two languages?
It can mean 'thing', but it's more common to use 'ting' when the meaning is 'thing'. So 'grønnsak' literally means 'green thing'.
kan man si på norsk(det er en sak av tid)som et eksampel it is a matter of time ?
takk skal du ha,can i ask sometimes questions that are not related to the lesson itself ,like asking about certain sentence that it is completely not on dulingo ,is it okay or rude?
If it's completely irrelevant to the sentence/lesson, I'd much prefer that you ask it on the Norwegian forum, or wait for a sentence/lesson where it would be relevant (if it's something you expect will come up).
The question you asked here is relevant enough though, as it deals with one of the words contained in the sentence.