Beginners question on rules syntax

I have some programming experience but not in Java or any of the derivative scripting languages. So for example when the Wiki mentions using JODA time it meant nothing to me.

So is there a reference document for the commands that can be used in creating rules?

Should I be looking at Xtend or Xbase? Where is the information on what needs to be in the import statements?

I have tried learning from the example rules but they often raise more questions than they answer. I hope the answer is not that to write rule you need to know Java.

I like Openhab but it has been hard work.