Does "spiser" become "is eating" when there is a subject just before it?
No, that's irrelevant. Norwegian only has on present tense, which covers both the simple present and the present continuous in English, so as long as the context allows for it "spiser" can be translated to either "eats/eat" or "is/are eating".
So when speaking you can distinguish them only according to the context then? like using time clauses or something??? I'm sorry I'm a native spanish speaker and I find it so hard to imagine it^^