**GRAMMATICAL STRUCTURE:**

> If you are to be a king, you need to look after these people. 

Here in this sentence the copular verb - *are* (auxiliary verb) - takes a non-finite *to*-infinitive clause (subordinate clause) as its complement. The clause is introduced by a subordinator - *to* (you can simply call it a *infinitive marker*). The Noun Phrase - *a king* - is the complement of the verb - *be* (main verb in the subordinate clause), and the implied subject of the subordinate clause is *you*. 

**SEMANTIC:**

> **1.** If you are to be a king, you need to look after these people. 

> **2.** If you are a king, you need to look after these people. 

In sentence #1, you are not yet *a king*, but if you are going to be a king you need to look after these people. 

In sentence #2, it suggests that you need to look after these people if you are already a king.