Subject vs. Object

Subject

When you write a sentence, you have to be clear. Your reader needs to understand what you are talking about. If you write something like: "Threw the ball". Your reader will start wondering, "what or who threw the ball?". It will happen because your sentence is not complete. However, if your sentence is "Michelle threw the ball", your reader will understand that the person Michelle did the action of throwing the ball. The word that was missing - Michelle - is the person who the sentence is about. This word is referred as a subject.

"Subject is a person, place, thing, or idea which the sentence is about. The action or event of the verb is done by this person, thing, place or idea." 

We know that a sentence is composed of words that are called parts of speech. Among these words, we have nouns and pronouns. A noun is a person, place, thing, or idea in a sentence. A pronoun is the word used in place of the noun. That being said, we can say that a subject can be a noun or a pronoun. 

Object

If the sentence was "Michelle threw", your reader will understand who did the action, but he won't know what was thrown. Nonetheless, if the sentence is "Michelle threw the ball", your reader will understand that the person, Michelle, did the action of throwing the thing, ball. In this case, the word that was missing - the ball - is the the thing that received the action of the verb. This word is referred to as an object. 

"Object is a noun or pronoun that receives the action of the verb, either directly, which we call direct objects, or indirectly, which we call indirect objects.

Object Direct and Object Indirect
In the sentence "Michelle threw the ball to Tim", we already know who is the subject, Michelle, and the object, the ball,  but we have one more piece of information: Tim. 

In this case, ball is the thing affected by the verb, that makes it the direct object. Tim is the person who benefits from the action of the verb, that makes Tim the indirect object. 

"The direct object is the person or thing affected by the verb."

"The indirect object is the person or thing who benefits from the action of the verb."

The sentence "Michelle threw the ball to Tim", could have been written as "Michelle threw Tim the ball" without changing the meaning.