Personal pronouns in Spanish are used almost the same way as their English equivalents. However, the main difference is that in Spanish they do not need to be expressed since the verb itself will identify the subject. This happens in English with the third person singular of the present tense (to think- thinks), where the -s shows us that we're dealing with a he, she or it, although in English the pronoun is never omitted with the exception of the imperative ("Sit down!", not "You, sit down!"), unless we want to emphasize on the person being addressed with the command. In Spanish, a pronoun is only needed when referring to a third person (singular or plural) for proper identification purposes since it can be a "she", a "he" or even a formal second person singular. In the plural, the verb ending could be referring to a "they" (masculine or feminine) or to "ustedes", a form of the second person plural used by everyone in all the Hispanic countries, except in Spain, where "vosotros" is preferred.
The "Vos" Pronoun
There is also another second person singular ("vos") used in some Latin American countries (especially the ones in the South Cone [Argentina, Chile, Paraguay and Uruguay], although in some Central American countries it is also used [Costa Rica, for example]). The conjugation for the "vos" in the present indicative differs from the standard second person singular (tú) in that the stress for the pronunciation lies on the last syllable, and not in the penultimate syllable: tú sabes - vos sabés (you know).
Direct Object Pronouns
First of all you must remember that a direct object in a sentence is the person, event or thing affected by the verb. The main difference between the use of the direct object pronouns in Spanish and English is their placement. While in English they substitute the direct object (and its article) and are placed where the original object was, in Spanish this pronoun is placed in front of the verb, replacing also any article used with the object previously.
Indirect Object Pronouns
An indirect object is usually a person receiving the direct object. The pronouns in Spanish are basically the same as the ones used for the direct objects, with the exception of the third person. It is important to remember that in Spanish, anytime that an indirect object is expressed, the pronoun must be present even if the indirect object is expressed in some other way (i.e., prepositional clause).
As you see, we have one that can be used only for the singular (le), one used only for the plural (les), and yet another one (se) that can be used for both! Nevertheless, the 'se' form is used only when the direct object pronoun is also used for reasons that seem to be primarily aesthetic (such as the use of the 'n' with the indefinite article in English: "an apple" vs. "a apple"). Although you'll see the pesky 'se' everywhere in standard writing in Spanish (i.e., newspaper articles, literature, manuals, etc.), you should be aware that there are many uses of 'se', and that it's not always used as an indirect object pronoun.
