Subject

From UniLang Wiki

The subject of a sentence is just that: the subject of the sentence, as a book has a theme. This is the substance of what is said. The subject is the nominative noun, the one that PERFORMS an action (unlike a direct or indirect object, to which or for which the aciton is done). Every sentence contains a subject.

For example:

The boy threw the ball to the dog. Since the boy is throwing, the boy is the subject.

The ball was thrown to the dog by the boy. This is the same sentence as above, only circuitous. In an active sentence (above), the subject is first, and the sentence is less wordy. In this example, the subject is last, and the speaker must use words like "was thrown" instead of "threw."

The dog caught the ball. The dog is the one doing the action (catching), therefore, it is the subject.

The ball was thrown to the dog. In this example, we don't know without context who threw the ball. Therefore, the subject is unannounced, "invisible," one could say.

The subject tends to be dropped in imperative sentences as well. For example:

Give me the ladder. The subject is not known in this sentence, but can be assumed, based on the speaker's direction of sight, hand motions, and other factors that language cannot convey.

Bill, give me the ladder. In this one, Bill is the subject... of course. Bill is the one who will give the ladder (if he so wishes).

Let's go swimming. In this one, "let's" is a contraction of let us. Who is "letting us" and who is the subject? Well, "we" are "letting ourselves," so "we" are the subject.