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:
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.