You might have one question: why is there no objectivity marking in the last phrase I am a physicist? Well, just look at it: you remove am because it is the action, and you are left with I and a physicist. You could say I am a physicist or a physicist am I, and, apart from the second phrase sounding weird, the meaning of the phrase does not change. That means that the sentence has no object, but two subjects instead. Thus, effectively, Esperanto does not mark objectivity with the word estas.
Yet, while phrases with estas have no object, most other verbs do, so don't forget to think about who the object is in a sentence. Try out a few phrases: