Subjects, Verbs and Objects
Subjects and objects are always nouns or pronouns. A subject is the noun that is doing the action (the verb). An object is the noun that is receiving the action, or having the action done to it/him/her. In a normal sentence, they are usually in the order: subject-verb-object. It is the same order in English as it is in Khmer.
For Example:
"He reads the book."
In this sentence, "he" is the subject because "he" is the one doing the action (reading). "The book" is the object because it is the one receiving the action (reading).
There can be multiple subjects and objects in each sentence, and there doesn't need to be an object for every verb.
For Example:
"Sophea drove to the store, where she bought rice."
In the first part of the example, "Sophea" is the subject because she is one that is doing the driving. But, there is nothing in the sentence that receives the action of driving, so there is no object. Sophea is driving a car or a motorbike. She's not driving a store.
In the second part of the example, the subject is "she," which is referring to Sophea, "bought" is the verb, and "rice" is the object. Rice is the object because it was the thing that had the action done to it. It was the thing that was bought.