Is the subject of sentence is action that takes place in sentence?

That's not quite right! The subject of a sentence is the person, place, or thing that performs the action.

Here's a breakdown:

* Subject: The person, place, or thing doing the action.

* Verb: The action itself.

* Object: The person, place, or thing that receives the action (optional).

Example:

* Subject: The dog

* Verb: barked

* Object: at the mailman

The action (barked) is performed by the subject (the dog). The object (the mailman) receives the action.

Important Note: Some sentences don't have objects, like "The sun shines." The subject (sun) performs the action (shines) but there's no direct recipient of the action.