I'm not a native so I could be wrong but from what I've learn it's:
- Den pene jenta - the pretty girl (definite singular feminine)
- Jenta er veldig pen - the girl is very beautiful
- Jentene er veldige pene - the girls are very beautiful (the last one I'm not sure if it's correct... I don't know if we also decline "veldig" here)
Because the adjective is not before "jenta" it just obeys to the gender and number of the substantive (jente). We don't use it in the definite form.
Once again I could be wrong, so if any native could correct this it'd be great.
As a native English speaker, I would say that "pretty" and "very cute" are close in meaning, but not interchangeable. "Cute" implies an attitude as well as an appearance; one would expect a genuinely cute person to be bubbly and maybe a little childlike (note: that's not the same as childish). A better synonym for "cute" might be "adorable". "Pretty" is a more neutral term. Someone can be pretty and cute, or pretty and serious.
A further distinction is that "cute" can apply to behaviour as well as just looks, while "pretty" cannot (I'm sure I could think of an exception, but it's not easy).
All that said, colloquially, the two words are almost interchangeable. But the distinction is strong enough that I honestly can't say which I think is more common when either could be used. So I'd recommend sticking with translating "pen" as "pretty" unless you have a good reason to do otherwise.