Hey, sorry for my late reply!
And not exactly. 'Weer' is just used more often than 'opnieuw'. In some sentences, it sounds really weird when you use 'opnieuw'. F.e:
We are going to the playground again! We gaan WEER naar de speeltuin = Common sentence! Wij gaan opnieuw naar de speeltuin = That's not what you want to use ;)
It's often used when you are busy doing something, you pause, and then do it again. For example going back to work, going to study again etc.
Addition: I typed a lot, but I deleted it. After some thinking, I discovered 1 thing :) Opnieuw is often used when something goes wrong! Like: Oh no, I have to do that level once more / again. I have to start over. In Dutch this would be: Oh nee, ik moet dit level opnieuw doen. Ik moet opnieuw beginnen. This is a sentence where 'opnieuw' is really in place and where 'weer' is a bit off.
I hope you can feel the difference. Even though it's still hard to explain, haha :)
Oh hey, I think I can see the difference now! The reason I wondered if it was formal or not is because I've seen it in the web browser on my phone when the page doesn't load correctly. It makes sense then that it was used because something went wrong, say the connection was broken, and it needs to load anew. Right?
In fact, having just typed that, I'm wondering now if "anew" is "opnieuw"'s English cousin, haha. You'd definitely say you'd start something anew, but wouldn't really go to the park anew. I think a belated penny just dropped.
Assuming I've got the right end of the stick, thanks so much for the helpful explanation!
I'd say it's because you need to keep the Subject+FiniteVerb+Object structure together. If you had had a non-finite verb (say, the sentence was using het Perfectum -you'd place the past participle, being a non-finite, at the end), then you'd place it after opnieuw.
We hebben het opnieuw gelezen.
Hope this helps.