Why you teach the word thick instead of broad?

The choice between "thick" and "broad" depends entirely on the context. There's no single reason why one would be "taught" instead of the other; it's about which word accurately describes the dimension being discussed.

* Thick refers to a large distance from one surface to its opposite surface (like the thickness of a book or a plank of wood). It implies depth or solidity.

* Broad refers to a large distance from one side to the opposite side (like the breadth of a river or a road). It implies width.

Therefore, if you're learning about the dimensions of objects, you'll learn both words because they describe different aspects of size. A teacher would choose the word that best fits the specific object and dimension being described. For example:

* "This book is thick." (Describing its depth)

* "This river is broad." (Describing its width)

There's no inherent preference or replacement; they're simply different words with different meanings.