The subject is the most important part in the sentence, it shows what the sentence is about. It denotes an object or a person that does the action. Often there may be different modifiers that go with the subject and make a noun phrase: The dog reached its paw. Some beautiful girls are dancing on the floor.
Usually, but not always the subject stands before the verb, as in:
The speeding bike crashed into a telephone booth.
Sometimes it can stand after the verb:
In a small house along the lake lives a family with ten noisy children.
It should be noted that the subject of a verb can never be a part of a prepositional phrase. Sometimes a prepositional phrase appears to be the subject or a part of it:
Neither of these boys wanted ice cream.
Normally we would assume that the boys are the subject since they are the performers of the action. But since they are a part of a prepositional phrase of these boys , the actual subject is the word Neither. Another example is:
My dog, along with her seven puppies, has chewed all of the stuffing out of the cushions.
Again, since along with her seven puppies is a prepositional phrase the subject is my dog.