How do we punctuate a subordinating adverb phrase or clause that comes before the main clause?

We put a comma after the subordinate clause.

Explanation:

When a subordinating adverb phrase or clause comes before the main clause of a sentence, the subordinate clause should be followed by a comma to separate the subordinate clause information with the independent's clause information. Here are some examples:

After the walk, we went to the shopping mall (Adverb phrase underlined).

In total silence, she waited (Adverb phrase underlined).

Before you leave, could you please shut the door? (Adverb clause underlined)

Since you do not care about the class, you may leave the room (Adverb clause underlined)

Notice that the difference between adverb phrases and clauses is that clauses always have a subject and a verb while phrases do not.