"It is a moose."
Translation:Det är en älg.
If we use "den" here, we are being very specific that that very moose right there, is a moose. Using "det" is just expressing that there's a moose.
- Det är en älg i skogen. (There's a moose in the forest.)
- Den är en älg. (That one is a moose.)
This must feel very complicated to a non-native speaker. I think you will get a lot better feel for the differences between these two with practice. This one of those weird things that a native speaker doesn't even think about.