How to understand difficult irules

There are a huge number of articles here on devcentral explaining iRules. What have you looked at so far ?

https://devcentral.f5.com/wiki/iRules.HomePage.ashx

https://devcentral.f5.com/irules

I am assuming you have completed reading the iRule 101: https://devcentral.f5.com/wiki/iRules.iRules101.ashx

Quite a few of the iRules use the event HTTP_REQUEST. Understand the different scenarios to use it.

Try to read and understand the 20linesorless iRule series: https://devcentral.f5.com/wiki/iRules.20LinesOrLess.ashx

Like IanB said before, if you don't understand with dificult iRule just see it -> https://devcentral.f5.com/wiki/iRules.iRules101.ashx