InfoPath tries to help out with XPath via the UI, but sometimes to be able to accomplish complex conditions, you may need to write your own XPath. A good knowledge of XPath is also useful for qRules commands.
Here are a few online resources I like for getting comfortable with XPath:
http://zvon.org/xxl/XPathTutorial/Output/examples.html
http://www.whitebeam.org/library/guide/TechNotes/xpathtestbed.rhtm
https://www.w3schools.com/xml/xpath_intro.asp
http://msdn.microsoft.com/en-us/library/ms256086.aspx
About Hilary Stoupa
I wandered into development after working as a business process analyst for a global manufacturing company. I create InfoPath solutions for our clients as well as work as a developer on company tools that extend InfoPath. I've also been instrumental in creating the InfoPath Master Class training provided by Qdabra.