Recognize a subject of a sentence when you see one.
During his biology lab, Tommy danced on the table.
Danced is an action verb. Tommy is who did the dancing. Look at the next example:
The speeding hotrod crashed into a telephone pole.
Crashed is the action verb. The hotrod is what did the crashing.
Not all verbs are action verbs. Some verbs are linking: am, is, are, was, were, seem, and become, among others. Linking verbs connect the subject to something that is said about the subject. Take a look at this example:
Ron's bathroom is a disaster.
Bathroom is the subject. Is connects the subject to something that is said about it, that the bathroom is a disaster. Here is another example:
The bathroom tiles are fuzzy with mold.
The word tiles is the subject. Are connects tiles to something said about them, that they are fuzzy with mold.
Generally, but not always, the subject of a linking verb will come before the linking verb.
Know the difference between a complete subject and a simple subject.
The complete subject is who or what is doing the verb plus all of the modifiers [descriptive words] that go with it. Read the sentence below:
The big, hungry, green Martian grabbed a student from the back row.
Who did the grabbing? The Martian, of course. But this Martian wasn't petite, satisfied, and blue. No, this one was big, hungry, and green. The complete subject, then, is the huge, hairy, hungry, green Martian.
The simple subject, on the other hand, is the who or what that is doing the verb without any description. Take a look at this example:
The bright copper coin sparkled on the sidewalk.
What did the sparkling? Obviously, the bright copper coin. The, bright and copper, however, are just description that distinguishes this coin from one that is, let's say, tarnished and silver. The simple subject is only the word coin.
Remember that the subject is never part of a prepositional phrase.
The subject of a verb will never be part of a prepositional phrase. A prepositional phrase begins with a preposition [in, on, at, between, among, etc.] and ends with a noun, pronoun, or gerund. Look at these examples of prepositional phrases:
in the dirty bathtub
on the bumpy road
among the empty pizza boxes
Sometimes a prepositional phrase appears to be either the subject itself or part of the subject. Read the example that follows:
Neither of these boys wants to try a piece of pineapple pizza.
In this sentence, the boys seem to be the ones who do not want the pizza, but because they are part of a prepositional phrase, of these boys, they are not the subject. Neither is the actual subject. Take a look at another example:
My dog, along with her seven puppies, has chewed all of the stuffing out of the sofa cushions.
Here, both my dog and her seven puppies are chewing on the sofa, but because the puppies are part of the prepositional phrase along with her seven puppies, the only word that counts as the subject is dog.
Remember this additional point:
Generally, but not always, the subject comes before the verb, as in all of the examples above. There are, however, exceptions, like this one:
In a small house adjacent to our backyard lives a family with ten noisy children.
Lives is the action verb in this sentence, but it is not the house or the backyard that is doing the living. Instead, it is the family with ten noisy children. Family, then, is the subject of this sentence, even though it comes after the verb. Take a look at another example:
Around the peach trees are several buzzing bumblebees.
Are is the linking verb in this sentence. The word trees, however, is not the subject because trees is within the prepositional phrase around the peach trees. The subject in this sentence, bumblebees, follows the verb rather than coming before it.