A pronoun is a word that can substitute for a noun or a noun phrase.

*They as a Singular Pronoun

Although it may seem grammatically incorrect, singular “they” has been in widespread use for several centuries appearing in the writings of Chaucer, Shakespeare, Austen, Woolf, and many other major English authors. Visit the APA or MLA's websites for formatting specifics. 

Please Note: When used as a singular pronoun, they, the verbs are conjugated the same as with the plural they (e.g. “they are”). Also note that in this singular pronoun set may use themself rather than themselves, although both are typically acceptable. Usually, the “they/them/theirs” pronouns set is acceptable to use when you do not have more information about the person.

Example:

• I don't know who the writer is, but they have a fantastic command of language.
• I'm trying to call a new mechanic, but they haven't picked up the phone all day!
• I haven't met Colby yet.  They signed up for my class just before the deadline.

Who

Should be used when the pronoun is the subject. 

• CORRECT: Who is able to join us? He is able to join us.

Whom

Should be used in questions when the pronoun is the object of the verb or preposition.

• CORRECT:  Whom did you meet for dinner?  I met them.

Whose

Expresses possession.

• CORRECT: I do not remember whose choice that was.

Reflexive pronouns are used when the subject and the object are the same. It can also be used for emphasis.

• CORRECT: I'll do this assignment myself.

Use the objective forms of personal pronouns instead of using the objective forms.

• INCORRECT: Jeff is going to meet up with Troy and I.
• CORRECT: Jeff is going to meet up with Troy and me.

An easy way to recognize when to use the objective form is to omit the noun proceeding the pronoun.  

For example, you wouldn't say:

• INCORRECT: Jeff is going to meet up with I.

The ambiguous use of pronouns as the subjects of sentences can be confusing to readers.

For example:

• AMBIGUOUS: Tina and Louise went to school, even though she didn't want to.

It is unclear who she refers to.  To correct this error, make it clear who she is.

• CLEAR: Tina and Louise went to school, eve though Tina didn't want to.

Some paragraphs contain sentences that are ambiguous and require clarification.

This can recreate confusion.

• AMBIGUOUS: Pens come in black and blue ink.  This is what you can use on your test.
• CLEAR: Pens come in black and blue ink.  You can use either when taking your test.

You should only be used when speaking directly to the reader.  

• AMBIGUOUS:  Dentists recommend that you floss at least once a day for healthy gums.
• CLEAR: Dentists recommend flossing at least once a day for healthy gums.

The use of gender-neutral language, while not specifically a grammatical issue, can be an important stylistic concern. It is good practice not to use either he or she as the default third person pronoun.

• NOT NEUTRAL: Each person has his preference as to what classes he adds to his schedule. 
• NEUTRAL: Everyone has a preference to what to put on a schedule.