Thanks. I've seen it but now that you mentioned I really liked to also understand what that means semantically:
5!:4 <'FB'
I see here:
http://www.jsoftware.com/help/dictionary/dx005.htm
That every combination of two numbers means something special.
http://www.jsoftware.com/help/dictionary/dx018.htm
Wow.
The only thing other than "practice" you can do is to use built-in J expression pretty-printer. It shows you a graph of an expression, grouping verbs with their arguments. I don't remember how to get to it exactly, I think it was somewhere in foreigns.
EDIT: it's even mentioned in the article, don't know how I missed it: 5!:4