Pronoun Placement

Subject Pronouns do the action of the verb:
I sing. [I is the subject pronoun (not optional in English); sing is the verb]
Yo canto. [Yo is the subject pronoun (optional in Spanish); canto is the verb]

Pronoun Objects of the Preposition are used after prepositions:
The gift is for me. [for is the preposition; me is the prepositional object pronoun]
El regalo es para mí. [para is the preposition, mí is the prepositional object pronoun]

Reflexive Pronouns (in theory) reflect the action of the verb (like a mirror) back onto the subject, so that subject and object are the same person:
I see myself in the mirror. [I is the subject pronoun (not optional in English); see is the verb (action); myself is the Reflexive Pronoun]
Yo me veo en el espejo. [Yo is the subject pronoun (optional in Spanish); veo is the verb; and me is the reflexive pronoun.]

Indirect Object Pronoun use in Spanish grammar is slightly different from English grammar. Indirect Object Pronouns present a way in Spanish to answer the question: to or for whom or what? without using either of the Spanish prepositions por or para (which can create synonymous structures without the use of the indirect object pronouns).
“I read the book to María” would translate into the Spanish “Yo (optional) le leí el libro a María”. In this sentence le is the grammatically required indirect object pronoun, whereas the prepositional phrase a María, although it provides more information and might be needed for understanding, is grammatically optional in the sentence.

Direct Object Pronouns receive the action of the verb:
I see it. [I is the subject pronoun (not optional in English); see is the verb]
Yo lo veo. [Yo is the subject pronoun (optional in Spanish); lo is the direct object pronoun corresponding to the word it in English; and veo is the verb].

Placement of Spanish Reflexive, Indirect, and Direct Object Pronouns.
If more than one of these three pronouns appears in a sentence, they will always be placed in this order:
Reflexive (1), Indirect (2), Direct (3)= RID. Examples: Pablo me lo compra. Pablo está comprándomelo. Pablo va a comprármelo. Yo estoy comprándomelo (a mí misma). ¡Cómpramelo! ¡No me lo compres!

Placement in a sentence of Spanish Reflexive, Indirect, and Direct Object Pronouns.

Reflexive Pronouns, Indirect Object Pronouns, and Direct Object Pronouns all follow the same rules of placement.
They are placed
BEFORE: AFTER:
1. Conjugated Verbs 1. Infinitives
2. Negative Commands 2. Gerunds (also called “Present Participles”--i.e., the
Spanish “-ndo” form that corresponds to the English “-ing”
3. Affirmative Commands

If more than one of these three object pronouns appears in a sentence (but usually you will only see any two of the three at one time), they will always be placed in this order:
Reflexive (1), Indirect (2), Direct (3)= RID.

EXAMPLES:
In the examples below, Pablo (the subject) is going shopping to buy a book (the direct object= a book/ it ; un libro/ lo).
BEFORE
1) Examples of Pronouns placed before conjugated verbs:
Pablo me compra un libro. “compra” is conjugated Pablo buy a book for me
(or Pablo buys me a book).
Pablo me lo compra. “compra” is conjugated Pablo buys it for me.
Pablo me lo está comprando. “está” is conjugated Pablo is buying it for me [right now].
Pablo me lo va a comprar. “va” is conjugated Pablo is going to buy it for me.

2) Example of Pronouns placed before a negative command:
¡No me compres un libro! Do not buy a book for me! (or Do not buy me a book!)
¡No me lo compres! Do not buy it for me!
***Note that all object pronouns placed before verb forms are written as separate words.

AFTER
1) Examples of Pronouns placed after an infinitive:
Pablo va a comprarme un libro. Pablo is going to buy a book for me.
Pablo va a comprármelo. Pablo is going to buy it for me.
Pablo va a comprarse un libro. Pablo is going to buy himself a book.
2) Examples of Pronouns placed after a gerund (present participle):
Pablo está comprándomelo. Pablo is buy it for me [implied: right now].
Yo estoy comprándomelo (a mí mismo/a). I am buying it for myself (reflexive) [implied: right now].

3) Example of Pronouns placed after an affirmative command:
¡Cómpramelo! Buy it for me!
***Note that when RID pronouns are placed before verb forms, they are written as separate words. When the object pronouns are written after the verb form, they must be attached, and added accent marks may be needed.
When to add accent marks:
Gerunds: An accent mark is added to a gerund whenever any pronoun is added to the gerund. The accent is always placed over the “a” or “e” immediately preceding the “-ndo” (Pablo está comprándome un libro. Pablo está comprándomelo ahora mismo.).
Infinitives: No accent needed if only one pronoun is added to an infinitive (Pablo va a comprarme un libro). An accent mark is added to the infinitive only when two pronouns are attached.
When two pronouns are attached to an infinitive, the accent is always placed over the “a”, “e” or “i” preceding the “r” of the infinitive (Pablo va a comprármelo; Pablo va a vendérmel;, Pablo va a pedírmelo).
Special Changes in the Indirect Object Pronouns le and les:
Whenever either of the Indirect Object Pronouns “le” or ”les” is used in combination with one of the Direct Object Pronouns that begin with the letter “l” (lo, la, los, las), the le or les changes to the same spelling as the Reflexive Pronoun “se”. The difference in meaning is usually clear from context.
Pablo les compra un libro a los niños. Pablo buys a book for the children.
Pablo se lo compra a los niños. Pablo buys it for the children.
**Note that the les in the first sentence changes in the second sentence to se because of the Direct Object Pronoun lo.