Question

larry couldnt remember where he had left his one clean pair of socks. bonzo knew that the footwear was buried in the backyard but since he was a dog, he couldnt tell poor larry that searching the house was a waste of time.

add a comma between remember and where.
add a comma between backyard and but.
add a comma between that and searching.
no change is necessary.

Explanation:

1. The first sentence is a complex sentence with a noun clause ("where he had left his one clean pair of socks") acting as the object of "remember"; no comma is needed between a verb and its object clause.
2. The second part is a compound sentence with the coordinating conjunction "but" joining two independent clauses ("Bonzo knew that the footwear was buried in the backyard" and "since he was a dog, he couldn't tell poor Larry that searching the house was a waste of time"). When a coordinating conjunction joins two independent clauses, a comma must precede the conjunction.
3. The clause "that searching the house was a waste of time" is a noun clause acting as the object of "tell"; no comma is needed between "that" and the start of the clause.

Answer:

Add a comma between backyard and but.