From Nouns to Pronouns
A subject may be a noun or a pronoun; basically, pronouns are words that are used to substitute for nouns: “you” can replace “the reader,” “his” may be used instead of “John Smith's,” and “them” might refer to “the students.” In English, “I,” “we,” “you,” “he, “she,” “it,” and “they” are known as subject pronouns. In Spanish, there are just a bit more pronouns to choose from, because there are multiple versions for that humble word “you.”
Personal Pronouns in English
Personal Pronouns in Spanish
Subject pronouns in Spanish and English are organized by person (first, second, or third) and number (singular or plural), but when you compare the two tables, you will notice a number of differences.
What is a grammatical person?
Basically, it's the point of view. First person is from the point of view of the speaker (I did this, we did that). Second person is from the point of view of the person being spoken to (you did that, why don't you …?). Third person is from the point of view of another person, neither the speaker nor the listener (he did this, they did that).
The most obvious difference is the additional third person category. Usted and ustedes are second-person pronouns because they translate as “you” and deal with the person being spoken to. However, in Spanish they are conjugated as third-person pronouns (more on conjugation later). That's because usted is an old contraction of vuestra merced (your mercy), a very formal address that was made in third person (just as in English you would say “your mercy wishes,” but “you wish”).
In the plural, only use nosotras, vosotras, or ellas when all members of the group are female (or, in the case of ellas, all objects represented are feminine-gender nouns). When referring to males or mixed groups, use the masculine versions of the pronouns. For example, las casas (the houses) are ellas, but los actores y las actrices (the actors and actresses) are ellos.