The subject is the agent of the sentence in the active voice; it is the person or thing that performs or is responsible for the action of the sentence and it normally precedes the verb.
The verb follows the subject in a declarative sentence; it generally shows the action of the sentence.
A complement completes the verb. It is similar to the subject because it is usually a noun or noun phrase; however, it generally follows the verb when the sentence is in the active voice.
A modifier tells the time, place or manner of the action. Very often it is a prepositional phrase. A prepositional phrase is a group of words that begins with a preposition and ends with a noun.
a. Jill is buying a new hat in the store.
(Jill = Subject | is buying = verb phrase | a new hat = complement | in the store = modifier of place)
b. George is cooking dinner tonight.
(George = Subject | is cooking = verb phrase | dinner = complement | tonight = modifier of time)
c. Henry and Marcia have visited the president in his house.
(Henry and Marcia = Subject | have visited = verb phrase | the president = complement | in his house = modifier of place)
d. We can eat lunch in this restaurant today.
(We = Subject | can eat = verb phrase | lunch = complement | in this restaurant = modifier of place | today = modifier of time)
e. She opened a checking account at the bank last week.
(She = Subject | opened = verb | a checking account = complement | at the bank = modifier of place | last week = modifier of time)
There are five forms of pronouns in english; subject pronouns, complement pronouns (object pronouns), possessive pronouns, possessive adjectives and reflexive pronouns.
1. Subject Pronouns
Subject pronouns occur in the subject position of a sentence or after the verb be.
Verbs: I, you, he, she, it, they, weExample: I am going to the store
2. Complement Pronouns
Complement pronouns occur in complement position, whether they complement a verb or a preposition.
Verbs: me, you, him, her, it, us, you, them
Example: They called us on the telephone.
3. Possessive Adjectives
Possessive adjectives are not the same as possessive pronouns. These simply modify, rather than replace, nouns; possessive pronouns replace nouns. Possessive forms indicate ownership.
Verbs: my, your, his, her, its, our, your, their
Example: John is eating his dinner.
4. Possessive Pronouns
These pronouns cannot precede a noun. They are pronouns and thus replace the noun. The noun is understood from the context and is not repeated.
mine = my + noun; for example, my book.
yours = your + noun; for example, your pen.
hers = her + noun; for example, her car.
Verbs: mine, yours, his, hers, its, ours, yours, theirs.
Example: This is my book = This is mine
5. Reflexive Pronouns
These pronouns usually follow the verb and indicate that the subject is both giving and receiving the action.
Verbs: myself, yourself, himself, herself, itself, ourselves, yourselves, themselves.
Example: John bought him a new car = John bought himself a new car.
Source: TOEFL: Preparation Guide by Michael A. Pyle, M. A. and Mary Ellen Munoz Page, M. A.