In high school, I had an HP-28S scientific calculator. It was a great calculator: you could graph functions, solve equations, store formulas, and write little programs for it. The way you did all the operations on the calculator was called ”Reverse Polish Notation”. It has nothing to do with the Polish language, just a Polish mathematician. If you’ve studied computer science, it is also called “postfix”. The basic idea is that the operator comes after the the operands. So to do three times nine, you would enter “3”, “9”, then ”*”. This might seem silly or pointless, but once you got used to it, it was so much easier to use. The results were stored on a stack that allowed you to do complicated calculations with ease.

Anyway, back to languages. In German, a verb often comes at the end of the sentence. The simplest case is with modal verbs (Michel Thomas calls them “handles”). When you use a modal verb, the second verb comes at the end.

Ich kann Klavier sehr güt spielen.

“kann” is the modal verb, “spielen” is the second verb. As soon as a modal verb is used, the RPN calculator starts up. Each word (operand) you see gets pushed onto the stack until you get to a verb (operator).

A phrase that uses “weil” also uses the stack/RPN calculator.

Ich gehe nach Hause, weil ich sehr müde bin.

“Weil” triggers the calculator, words are pushed on until you get to a verb to operate on the stack.

“Denn”, on the other hand, doesn’t use RPN:

Ich gehe nach Hause, denn ich bin sehr müde.

More complex phrases also follow this pattern.

Ich werde es kaufen, weil ich es haben möchte.
          stack 1         stack 2

I imagine this explanation of this part of German grammar just made it more confusing. But for those of you RPN/postfix aficionados learning German, I hope it will help!