THEORY

 
The following theoretical knowledge, among other, is covered in the sailing courses:

BOAT PARTS
SAILING TERMS
SAILING KNOTS
PRIORITY RULES

If you want to learn sailing, you need to get out on the water, feel the forces working on the boat , You need to put in many hours of practice to master it.
Theoretical knowledge is also important, and gives  you a better understanding  of sailing. 
You can learn a lot of knowledge in advance, on shore. 
Being on the water you  can then focus 100% on sailing itself. Knowledge of  theory beforehand is not necessary, but will certainly speed up your learning process during the course.