The subordinate clause is "When the delivery man arrives" because 1. it is introduced by "when" and 2. we can ask question to the main clause about this one.
A subordinating clause is a group of related words that have at least a subject and a verb, and that can not stand as a full sentence because it does not express a complete thought.
In the sentence provided, the subordinating clause is "When the delivery man arrives" because it has a subject: "the delivery man" and a verb: "arrives" but it does not express a complete thought on its own, it is not an independent clause.
As for the clause "ask him to put the boxes in the garage", it is an independent clause because it does express a complete idea.
