What is the subordinate clause in this sentence? "did you know that many people can't name the last five presidents? " !

Notice how "that many people can't name the last five presidents" can't stand on its own as a sentence because it begins with a subordinating conjunction, "that". The main clause in your sentence is "Did you know". A subordinate clause—also called a dependent clause—will begin with a subordinate conjunction or a relative pronoun and will contain both a subject and a verb. This combination of words will not form a complete sentence. It will instead make a reader want additional information to finish the thought.