[Grammar] Infinitive phrase as a Subject Complement

[hothead2692]

"They are to protect the leader at all cost"

I already know the infinitive act as a subject complement but what I really want to know is its function to the subject. Does the infinitive act as a noun, adverbial, or adjective?

 

[5jj]

What makes you think the infinitive is a subject complement in that sentence?

 

[hothead2692]

Because it comes after a linking verb. The sentence orginally came from the context of a captain ordering his squad to protect the leader.

 

[5jj]

It does not come after a linking verb. It come after the quasi-modal am/is/are to.