In addition to their various classifications, nouns pronouns have three major characteristics: case, number, and gender.
Noun and Pronoun Case
The case of a noun or pronoun determines how you can use it in a phrase or clause. There are three cases in Modern English (as opposed to eight in Classical Latin, four in German, and only two in French):
You use the subject case for a noun or pronoun which stands alone, is the subject of a clause, is the subject complement, or stands in apposition to any of these.
You use the object case for the object of a preposition, a verb, or a verbal, or for any noun or pronoun which stands in apposition to one of these.
You use the possessive case for any noun or pronoun which acts an an adjective, implicitly or explicitly modifying another element in the sentence.
Nouns always take the same form in the subject case and the object case, while pronouns often change their form. Both nouns and pronouns usually change their form for the possessive case:
The man travelled to Newfoundland.
He travelled to Newfoundland.
The taxi drove the man to the airport.
The taxi drove him to the airport.
The baggage handlers lost the man's suitcase.
The baggage handlers lost his suitcase.
For further information, see possessive nouns, possessive pronouns, and possessive adjectives.
Noun and Pronoun Number
The number of a noun or pronoun is either singular, if it refers to one thing, or plural, if it refers to more than one thing (if the noun or pronoun is the subject, then its number will also affect the verb). Note the difference in number in the following examples:
That woman is concerned about this issue.
She is concerned about this issue.
Those women are concerned about this issue.
They are concerned about this issue.
It is important to note that the pronoun "they" is in the processing of becoming singular as well as plural. For example, one might say
A person called and they did not leave their name.
This construction allows the speaker to avoid identifying the gender of a person, and it has been common in speech for decades, if not for centuries. Be aware, however, that some people still consider it unacceptable for formal writing.
For more information, see noun plurals.
Noun and Pronoun Gender
Unlike the Romance languages (such as French, Spanish, and Italian), English has three genders for nouns and pronouns: masculine, feminine, and neuter.
Generally, the English language uses natural gender rather than grammatical gender -- that is, the gender of a word is usually based on its biology (so there is little need to remember whether a word is masculine or feminine). A noun that refers to something with male sexual organs is masculine, a noun that refers to something with female sexual organs is feminine and most other nouns are neuter by default.
There was a time when you could use the masculine gender by default when you did not know a person's natural gender, but very few people accept this usage any longer.
There are, moreover, a few tricky points. First, you may refer to all animals in the neuter gender, or you may refer to them by their natural gender:
What a beautiful dog! Does it bite?
What a beautiful dog! Does she bite?
Second, You usually assign mythical beings (such as gods) to a natural gender, even if you do not believe that the beings have actual sexual organs:
God is great. God is good. Let us thank her for our food.
Finally, people sometimes assign natural gender to inanimate objects, especially if they live or work closely with them. When engineers were mostly men, for example, they tended to refer to large machines in the feminine:
She is a fine ship.
For more information, see the discussion of gender-specific nouns.
Noun and Pronoun Person
Personal pronouns always belong to one of three persons: first person if they refer to the speaker or writer (or to a group including the speaker or writer), second person if they refer to the audience of the speaker or writer (or to a group including the audience), and third person if they refer to anyone else (if the noun or pronoun is the subject, then its person will also affect the verb). Nouns and other types of pronouns are always in the third person. Note the differences in person in the following examples:
I will come tomorrow.
Bob showed the budget to us.
You should not forget to vote.
Where is your coat?
It arrived yesterday.
How can you stand working with them?
Traditionally, you were required to use the third person in formal academic writing, but some people now accept the first person. Whichever you choose, however, you must be consistent.