Answer:
Adverb clause
Explanation:
The sentence consists of two clauses: an adverb clause which it's “After a long day of hard work was over” and that we can identify as such because it begins with the subordinating conjunction “After” and it provides information about time; and an independent clause: “Tom went to the lake” which is also the main clause because it expresses a complete idea.
Adverb clauses may be placed at the beginning, middle or end of the sentence. The grammar rule holds that when an adverb clause begins a sentence, a comma must follow, just like it is in the sentence. Therefore, the function of the introductory comma of the sentence is to separate the adverb clause from the main clause.
