Need help with 中 vs 中に

This is the only grammar point I’ve had significant trouble with s far - the Bunpro description does not clear it up for me at all. When is に used? And, when is 中 pronounced as じゅう? Thanks :’)

Heres an answer for the latter part of your query:

Check out the other resources, maybe it will cast some light on your questions. MaggieSensei is usually VERY thorough in her explanations and is where I would start.