When talking about something in an abstract or general sense, the -t form is used.
Then how to define such "general sense"? If it is not practice, but computer skills practice,can it still be general sense or specific?
Yes. The question is: Do you want to say that some particular exercise is important or do you want to say that it is important to exercise in general. In the former case you would say En övning är viktig. In the latter case you say Övning är viktigt.
A bit like 'Practice is something important', 'Practice is an important phenomenon'? That would make it sound pretty abstract and neutral, which goes with the -t ending.
the reason mine is funny is that the set expression in Swedish is övning ger färdighet 'practice gives skill' which sounds a lot like my more sinister version but has a totally different meaning.
Not quite - mine means "practice makes practice" and Arnauti's means "practice makes horrendousness". :p
If övning refers to formal learning repetition, what is the word for physical exercise, like going to the gym or going for a run?
This sentence just after "There are six classes in the school"… Duo, you made my day! XD
Typically jag övar/tränar på [noun] or jag övar/tränar på/för att [verb].