"It is me."
Translation:Det er mig.
I'm just a learner too, but I have found a difference in the negation which may shed light on what is happening. The negation of this sentence would be "det er ikke mig" (it is not me); whereas "det er jeg ikke" means approximately "I am not that" or "that is not what I am" (note also the different position of "ikke"). Perhaps a native speaker could also weigh in here?
I think you pretty much always use the object form just like in colloquial English. People are constantly trying to claim that in English you should use the subject as you do in Latin and German and other languages. But it's not really true. I think it works like stress pronouns in French.
Who is it? It's me. -is analogous to- Qui est-ce? C'est moi. -- Hvem er det? Det er mig.
When English speakers insist on using the stiff sounding 'It is I', they are merely attempting to apply Latin rules to a language where they don't really fit very well. In German or a Slavic language, they work well. (Das bin ich; Ich bin's -- although, really, I guess 'ich' is the subject of these sentences as signaled by the verb 'bin') But not in English, French, Danish, Norwegian, Swedish.