[kizzyb]

The shoreline/ has many different shapes.

What is the subject and the simple subject. Trying to help my child. But I need understanding also.

[valinors_sorrow]

Shoreline is the subject and simple subject both, I believe. It looks like something of a trick question by inviting you to pick "shapes" as the simple subject and "different shapes" as the complete subject but in fact that is an object modified by an adjective.

I think it works like this (if I remember my grammar lessons properly)
Here is an example sentence:

"A piece of pie would be nice."

"A piece of pie"-- is the whole subject

"piece"-- is the simple subject being modified by "A" and "of pie"