Identity / role
I am a volunteer. · “A volunteer” names the role; a singular role noun takes a.
To Be
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I am a volunteer. · “A volunteer” names the role; a singular role noun takes a.
I am nervous. · “Nervous” describes a state; no article comes before the adjective.
I am near the entrance. · The place phrase locates the subject.
Same subject + same be form; a different complement changes the meaning. What stayed the same? What changed the meaning?
El contexto decide