An adverb clause is a subordinate clause that describes the verb of the sentence.
The main clause of the sentence is “I like tomatoes.” This clause is the independent clause because it has a subject, a verb, and a complete thought. It can stand alone as its own sentence.
The adverb clause of this sentence is “when they are placed on a hamburger.”
We know it is an adverb clause because it describes the verb like, and tells when. Adverbs tell when, where, and in what way.
The subject of the adverb clause is “they,” and the antecedent of “they” is tomatoes. The verb of the adverb clause is “are placed.”