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The basic parts of a sentence fall into two cats: the subject and the predicate. Know them well, because you can’t have a sentence without them!
Subjects
The subject performs the action of the sentence. It can be a noun, noun phrase, or noun clause. To analyze sentences, grammarians have distinguished three kinds of subjects:
- Complete Subject - "The old, dusty books on the table haven't been read in years." The complete subject includes the entire noun phrase -- usually everything before the verb.
- Simple Subject - "books". The simple subject is the main noun or pronoun stripped of all modifiers.
- Compound Subject - "The cowardly mailman and the huge, barking dog didn't get along very well." A compound subject consists of two or more subjects linked together by conjunctions. Note: the simple subject of that sentence would be "mailman and dog".
See Sentence Subjects for a closer look at subjects and subject-verb inversion (placing a subject after the verb in a sentence, as in “How is Bob?”).
Predicates
As with subjects, predicates can be classified as complete predicates, simple predicates, and compound predicates; see this page in the UIUC Grammar Handbook for more. To form a complete sentence, the predicate must include a verb (a finite verb, more specifically). It can also include objects, complements, and adverbials.
Objects
The object is the receiver of the action in a sentence: “He broke the table” or “He threw the ball.” Like subjects, objects can be any word or group of words functioning as a noun, and each type of object can also be categorized as a complete, simple, or compound object. Categorized by their different functions within a sentence, the three types of objects are:
- Direct Object - "I wrote a letter." (What did I write? A letter.)
- Indirect Object - "I wrote a letter to my friend." (Who did I write a letter to? My friend.)
- Prepositional Object - "I wrote on the paper." (What did I write on? The paper.) A thread on EnglishForums.com discusses prepositional objects and their potential for confusion.
Complements
Complements (also called predicatives) complete the predicate by modifying a noun in the sentence; copulas or linking verbs require a complement to form a complete sentence.
- Subject Complement - "The car is new." The subject complement follows a linking verb and modifies the subject. It can be a predicate adjective (He is happy), a predicate noun (He is the boss), or an adverbial complement (He is in the house).
- Object Complement - "I painted my room purple." The object complement modifies the direct object, either by describing it or renaming it (They elected him governor). Object complements can cause some confusion; check out this Pain in the English post. Also see Wikipedia's note on Object Complements.
- Adjective Complement - "He was happy to help." The adjective complement is a special case in which a group of words modifies an adjective. If removed, the adjective complement leaves a grammatically complete sentence, but the meaning of the sentence changes. The CCC Guide to Grammar and Writing explains the use of infinitive phrases as adjective complements. Note: predicative adjectives are also sometimes called adjective complements.
- Verb Complement - Some grammarians use the term "verb complement" to refer to direct and indirect objects (see the "Objects" section above), while others use it to refer to a complement occuring after a linking verb (a subject complement).
Adverbials
An adverbial is an adverb, adverbial phrase, or adverbial clause: any word or group of words that acts as an adverb within a sentence. They usually modify verbs, but they can also modify the whole sentence. Unlike an adverbial complement (He is in the house), an adverbial isn’t needed to complete a sentence (He had lunch in the house or He had lunch).