Question

What is the simple subject in this sentence "Bobby swam in the lake over the summer.

Answer 1

Bobby is the simple subject

Answer 2

I believe your answer would be Lake.

A simple subject completes the sentence, and the sentence can't be without it.
In this case, the sentence would be "Bobby swam in the over the summer." It doesn't make sense right? So with that example, I'm pretty certain the answer is Lake :)

Hope this helps!