A person teaches. But, a person IS NOT a teacher.
A person writes. But, a person IS NOT a writer.
A person thinks. But, a person IS NOT a thinker.
A person breathes. But, a person IS NOT a breather.
When I am teaching a class, then I am a teacher. But, it is merely a contingent, situational role. Teaching does not define the totality of who I am.
It's very important that we don't define ourselves in totality by what we do. Because, if we do, we set ourselves up for depression. How so?
let's say you play baseball for a living, and your whole identity is defined as "a baseball player." Then, suddenly, you seriously injure yourself and can't play baseball anymore.
A person who identifies himself as a baseball player may consider his inability to play baseball as the death of himself.
The reality is that we possess many identities, and psychological/spiritual danger exists only when a person over-identifies with one identity, as if one identity defines the entirety of his or her being.
Of course, the over-use of the "is of" language structure tends to push us into thinking that we have one essential identity that defines the core "I."