Step 1: Identify the components of the active sentence. Subject: 'You', Modal verb: 'must', Main verb: 'hit', Object: 'the nail', Prepositional phrase: 'on the head'.
Step 2: Form the passive structure. The object of the active sentence ('the nail') becomes the subject of the passive sentence.
Step 3: Change the verb form. The active modal verb ('must hit') changes to the passive modal form ('modal + be + past participle'). This becomes 'must be hit'.
Step 4: Construct the final sentence. The complete passive sentence is: "The nail must be hit on the head."