When constructing sentences in the English language, terms like the subject, object, and predicate, are included, but the problem arises when people don't know how to state what each word stands for. For example, John slapped Janet over a trivial issue. From this example, it will be very difficult to identify the subject and the object if you are just seeing the two terms for the first time. The subject in any sentence simply refers to the person or thing performing an action. In other words, it means the doer of an action.
From the example given above, John is the subject in that sentence because he slapped Janet. On the other hand, an object in any sentence is the receiver of an action. From the example given above, Janet is the object of that sentence because she got slapped by John. Prepositional object, direct object, and indirect object are different ways by which the object of a sentence can be classified.