Answer:
a subject
Step-by-step explanation:
The second sentence presented in the sentence above, needs the presence of a subject to become a complete sentence.
The subject of a sentence is the word that is executing, or suffering, the action of the verb of that sentence. As you may already know, the verb is the word that indicates an action or a state. In the case of the phrase "Did card tricks" the verb is the word "did," but we do not know who is promoting this action, for this reason, we can say that this phrase needs a subject, who indicates who is "making" the cards tricks.