In the sentence: "The boy kicked the ball," the ball is obviously the direct object.
A student asked me if the sentence were changed to "The boy kicked nothing," is nothing also a direct object?
- The boy kicked nothing
A good test for objects is to see whether the phrase can become the subject in a passivised version of the sentence. For the Original Poster's example we get the following sentence:
- Nothing was kicked by the boy.
Here we see nothing in subject position. We can do another little test just to check if this is the subject. If we make a question, does this word change places with the auxiliary verb was:
- Was nothing kicked by the boy?
Yes, nothing and was changed positions here. Nothing is definitely the subject of the passive voice sentence. This shows that it was the object in the active voice sentence.
A direct object that is a noun can be replaced by a pronoun.
- I like my teacher. I like him.
It can even be replaced by indefinite pronouns such as something, nothing, everything etc.
- I understand the text. I understand it. I understand everything/nothing.
Probably you wouldn't have doubts whether nothing can be a direct object if it were spelled as two words: I understand "no thing".