Consider the English sentence Cats eat mice. In the situation described by this sentence, there's a clear difference in the role played by cats and that played by mice. One ends up with a full belly, and the other ends up eaten! The sentence Mice eat cats, on the other hand, describes a situation where the roles are reversed: here, the mice have full bellies and the cats end up as dinner. How do we, as speakers of English, tell the difference between these two situations?
The only difference between Cats eat mice and Mice eat cats is the order in which the noun phrases1 appear in the sentence. In typical sentences like these, we call the first noun phrase the subject and the second noun phrase the object.2 The subject precedes the main verb, and the object follows it. Subject and object are two of the most basic grammatical relations that noun phrases can participate in. In English, the order in which noun phrases appear within the sentence allows us to distinguish between the two.
Let's take a look at another way that English distinguishes between subject and object. Consider the sentence Dogs chase my cat. Here we have the subject dogs and the object my cat. Now let's reverse the roles, and look at the sentence My cat chases dogs. Now dogs is the object, and my cat is the subject. As before, we see that the word order has changed to reflect the new grammatical relations. But something else has changed as well: chase has become chases. Why? Because, in English, verbs must agree with their subjects.3
Now we have collected two properties typically associated with subjects in English: they occur before the main verb, and the main verb agrees with them. We also have found a property of objects, namely that they follow the verb. These properties are specific to English. In other languages, subjects can be identified in other ways. For example, in Irish, the subject follows the main verb:
Itheann an bhean an t-úll. eats the woman the apple The woman eats the apple.
Let's now think about how the grammatical function of a noun phrase relates to the role played by the referent of the noun phrase4 in the situation the sentence describes. Let's return to Cats eat mice. We might be tempted to say the subject (cats) is the thing that "does the action" and the object (mice) is the thing that gets the action done to it. This description holds for Cats eat mice. But, in fact, there isn't a tidy relationship between the grammatical function of a noun phrase and its semantic role in the sentence.
To prove this, let's look at a passive sentence like Mice are eaten. Here the grammatical relations have changed: now mice is the subject. We know because mice occurs before the main verb, and the main verb are agrees with mice. It's Mice are eaten, not *Mice is eaten. But mice are definitely not the "doers" of the eating. This shows us that the notion of subject is related to, but independent of, any semantic concepts. We need to know what kind of sentence we're dealing with in order to know how to relate the grammatical function of a noun phrase to its semantic role in the sentence. In active sentences like Cats eat mice, the subject noun phrase refers to the thing that performs the action. But in passives, the subject noun phrase refers to the thing that undergoes the action.
To sum up our discussion, subject and object are grammatical relations that noun phrases can participate in. Grammatical relations are not defined semantically, but rather grammatically, by the way they relate to other elements within the sentence. For example, where do they appear? Do they trigger agreement? Finally, the grammatical properties that allow us to identify subjects and objects, such as word order and agreement, vary from language to language.
A noun phrase is a group of words that contains at minimum a noun (or pronoun) and functions as a noun. Some examples include Elaine, my friend, the apartment across the hall. ↩
More specifically, the second noun phrase in Cats eat mice and Mice eat cats is the direct object. There are also indirect objects, which represent the beneficiary or recipient of an action. These only occur in sentences which also have a direct object. For example, in the sentence I gave him a book, him is the indirect object while a book is the direct object. ↩
The only places where we see this agreement are (a) when the subject is a third-person singular form and the verb is in the present tense; or (b) if the verb is irregular, such as be and have. ↩
The referent of a noun phrase is simply the thing that the noun phrase refers to. ↩