Standard English requires knowing about nouns and pronouns, specifically, what their functions are in a sentence. To get to the subject of subjects, we have to do a little work with the concept of the sentence, and the role of verbs:
What is a sentence? The sentence is the basic unit of speech, because (by definition) it expresses a complete thought. It will not be a sentence — that is, the thought will not be considered to be complete — unless it contains both a “subject” and a “predicate.” A string of words that does not include a subject and a predicate is called a “sentence fragment.”
The central figure in the subject is a noun, either a person or a thing. The main element of the predicate is a verb, either in active or passive voice. The subject names who or what it is that either takes the action described in the predicate (active voice), or suffers the action of the predicate (passive voice). Here are two examples:
The dog was eaten by the lizard.
In both examples, the subject is “the dog.” In the first example, “ate the lizard” is the predicate, and in the second example, “was eaten by the lizard” is the predicate. The meanings, of course, are totally different.
The first example is “active voice,” and the active noun (the noun that does what the verb describes) is “dog.” “Dog” is the actor. Some grammarians call that “the agent.” (“Agent” comes from Latin for “doer.”)
The second example is “passive voice” and the passive noun (the noun on which the action of the verb is performed) is “dog.” “Dog” is the one who suffers the impact of the action. Some grammarians call that “the patient.” (“Patient” comes from Latin for “sufferer.”)
If you have trouble keeping these concepts straight, think of Mark Knopfler’s song (sung by Dire Straits and also by Mary Chapin Carpenter), “Sometimes you’re the windshield, sometimes you’re the bug.” The windshield is the “agent” and the bug is the “patient.”
Why does this matter? What does it have to do with subjects and objects?
Well, many active verbs have “objects.” If the subject is the agent (because the verb is in active voice), and if there is a patient (on which the action of the verb is taken), that patient is a “direct object.” The “ob-” prefix in Latin means “in the way of,” and an “object” is right in the path of the verb’s action.
So . . . why does knowing the difference between subjects and objects matter? If you were speaking German, Russian, Greek or Latin, for example, the noun you choose for an object will have a different grammatical form from that used for a subject. But you are lucky. This is English. We do not add endings according to the grammatical function of nouns (subject or object). We used to do that, but not in modern times. There is a little bit of bad luck: We still change the forms of some pronouns, according to their use as subjects or objects.
We call the subject form of these changeable pronouns the “nominative” case or form. Why? “Nominative” comes from the Latin word for “name,” and the subject “names” the actor (or the recipient of the action in the case of passive voice). We call the object form of these changeable pronouns the “objective” case or form, for obvious reasons. (Sometimes grammarians will call the objective case the “accusative,” which is a borrowing from classical languages.)
Consider two similar sentences:
I love you.
You love me.
In the first sentence, “I” is the subject and “you” is the object. In the second sentence, the roles are reversed, and “you” is the subject, and “me” is the object. From this you can conclude that the first person singular personal pronoun (“I/me”) changes form according to its function as a subject or as an object. You can also conclude that the second person singular (and plural) personal pronoun (“you”) does not change form according to its function as a subject or object.
The other pronouns besides “I/me” that change according to whether they are subjects or objects are: “he/him”, “she/her”, “we/us”, “they/them.” Exempt from this issue are “it” and “you.”
Curiously, among all the other types of pronouns (relative, possessive, interrogative, demonstrative) the problem arises only with “who/whom.” All the others (for example, “which”, “that”, “this”) do not change form between the nominative and the objective cases. The possessive pronouns and adjectives are neither nominative nor objective — they are their own type: possessive. They are already in the correct form.
So far, things have remained fairly simple: In the case of just six pronouns (I, he, she, we, they, who), it is important to know when they are used as subjects and when they are used as objects, because the nominative and the objective forms are different. In all other cases — all the other pronouns and all the nouns (both common and proper) — the same form is used for the nominative and the objective.
Three refinements are necessary at this point:
First, the direct object of a sentence (or clause) is not the only time the objective (or accusative) case is used. It is also used for the objects of prepositions. Therefore, we say “with me,” not “with I.” We should say, “to whom” and not “to who.” Usually it is not difficult to tell if a pronoun is the object of a preposition, so this rule is not very hard to follow.
Second, the objective case is also used with indirect objects. In the sentence, “Call me a cab” the direct object is “cab,” the noun that receives the impact of the verb call. In this case, “me” is the indirect object, the person or thing to whom or for whom the action is performed. Another example: “I gave her a ring.” The direct object is “ring”; “her” is the indirect object, and it is also in the objective case.
Third, some verbs do not even have direct objects. Intransitive verbs, by definition take no direct object. Examples: “He woke up” and “I get around.” That should not pose a problem: If the verb takes no direct object, then there’s no agony of choice of form.
Other verbs, however, do not take direct objects because they are not “action” verbs, but rather, “state of being” verbs. They are more like equals signs in the sentence. That is why they are sometimes called “linking verbs.” The verb “to be” (am/are/is) is the most common state of being verb. Others include “become,” “remain,” “seem,” “feel” (as in “the carpet feels good”), “taste” (as in “the food tastes good”), and “smell” (as in “the rose smells good”). (Note that “I don’t feel well” is correct: “well” in this case is an adjective meaning “healthy”; it is not the adverb form of good, which, of course, would not be correct in this situation.)
To be doubly clear: Consider “I am a student.” The verb “to be” (“am”) does not transfer action, it simply defines: I = student. In that case, “student” would be in the nominative case, to match its subject, the pronoun “I.” So … now the tricky one, in which one of the six changeable pronouns is in the predicate of a state-of-being verb: “It is I” is correct; “It is me” is incorrect. Do not say “It’s me”. Likewise, “This is she” is correct. “It’s her” is incorrect. The wrong forms are not even arguable. Predicate pronouns are the principal cause of trouble in keeping subjects and objects in line. This is because predicate pronouns are not objects. Like all pronouns, they “take the place of a noun.” In this instance, they take the place of a noun that happens to be the subject of the sentence, so they must take the same case as the subject: nominative.