why is "hun overvejer den" wrong? What if "it" is a "en" word?
If the noun isn't specified, it's 'det'.
We don't know what it is, so it defaults to det.
why is er not in the sentence?
Overvejer means "consider" or "is considering". The progressive form (or -ing form) is being used abundantly in English, but in most other languages its equivalent is only used to describe that someone doing something for a stretch of time.
Hun betragter det?