Questions & Answers about Het nieuwe ontwerp is deels modern en deels historisch.
Do I have to use en between the two deels phrases?
In a simple list of adjectives like this one, yes, you usually connect them with en. However, if you are connecting two longer clauses, you might omit en and just use a comma: Het is deels een succes, deels een mislukking (It is partly a success, partly a failure).
Could I use gedeeltelijk instead of deels here?
Yes, gedeeltelijk means the exact same thing. You could say gedeeltelijk modern en gedeeltelijk historisch. Deels is just shorter and often preferred when using this paired structure.
Why does nieuwe get an -e at the end, but modern and historisch do not?
Nieuwe comes directly before the noun (ontwerp). Because ontwerp is a het-word with a definite article (het) in front of it, nieuw gets an -e. Modern and historisch come after the verb is (they are predicate adjectives), and predicate adjectives in Dutch never get an -e ending.
If I started the sentence with the first deels, what would happen to the word order?