Lesson 54 introduces a new sentence structure.
The new structure is based on a cause, a linking word - therefor, and a result.
"It was raining, therefor the soil was wet".
(A mirror structure of such sentence was introduced in lesson #9:
"The soil was wet because it was raining".)
Therefor="LaChen".
Also, lesson 55 provides a good review of past tense rules of Hitpael structure.