What is the difference between direct and indirect object pronouns in Spanish?

Direct objects receive the action of the verb; indirect objects receive the direct object. In 'I give the book to María', the book is the direct object (what is given) and María is the indirect object (who receives it). Spanish direct object pronouns are me, te, lo, la, nos, os, los, las; indirect object pronouns are me, te, le, nos, os, les. The first and second person forms (me, te, nos, os) are identical for both; the third person forms diverge.

Why does le lo become se lo?

Spanish doesn't allow two consecutive object pronouns starting with l (le lo, le la, le los, le las, les lo, etc.). When the indirect object pronoun le or les is followed by a third-person direct object pronoun (lo, la, los, las), le or les becomes se. So 'I give it to him' is se lo doy, not 'le lo doy'. It's a pure phonetic fix, nothing to do with the reflexive se.

Spanish Object Pronouns

Spanish pronouns shift to before the verb. Veo el libro stays SVO, but lo veo flips to Object-Verb. Internalising this shift is the single biggest move between A1 and B1 word order.

The two sets

Direct object pronouns replace the thing or person the action is done to.

Person	Direct
me	me
you (tú)	te
him / it (m)	lo
her / it (f)	la
us	nos
you (vosotros)	os
them (m)	los
them (f)	las

Indirect object pronouns replace the recipient of the action.

Person	Indirect
me	me
you (tú)	te
him / her / it / you-formal	le
us	nos
you (vosotros)	os
them / you-formal-plural	les

Note the overlap. me, te, nos, os are identical for direct and indirect. Only the third-person forms split: lo, la, los, las for direct; le, les for indirect.

Position: before the conjugated verb

The default position for any object pronoun is directly before the conjugated verb.

Veo el libro. (I see the book.) → Lo veo. (I see it.)
Compro la casa. (I buy the house.) → La compro. (I buy it.)
Doy el libro a María. (I give the book to Maria.) → Le doy el libro. (I give her the book.) → Se lo doy. (I give it to her.)

Negation goes in front of the whole pronoun-plus-verb cluster: no lo veo, no se lo doy.

Attachment to infinitives, gerunds and affirmative commands

With a non-finite verb form, the pronoun attaches to the end of the verb.

Infinitive: quiero verlo (I want to see it). Both forms work with an auxiliary: lo quiero ver and quiero verlo are both grammatical.
Gerund: estoy viéndolo (I'm watching it). Both forms work: lo estoy viendo and estoy viéndolo.
Affirmative imperative: dámelo (give it to me). Required - you cannot say "me lo da" as a command.
Negative imperative: no me lo des (don't give it to me). Reverts to pre-verbal.

Note the written accents on viéndolo and dámelo. Attaching pronouns shifts the stress pattern, and a written accent goes on the original stressed syllable to preserve it.

Order when both appear

When you have both an indirect and a direct pronoun, they stack in a fixed order: indirect first, direct second.

Me lo dice. (He tells it to me. me = indirect, lo = direct)
Te la doy. (I give it to you. te = indirect, la = direct)
Nos los compraron. (They bought them for us.)

The se lo rule

Spanish refuses to put two l-starting pronouns next to each other. When the indirect le or les is followed by a third-person direct lo / la / los / las, le / les becomes se.

Le doy el libro a María → Se lo doy (not "le lo doy")
Les compré las flores → Se las compré (not "les las compré")
Le digo la verdad a él → Se la digo (not "le la digo")

The se here has nothing to do with reflexives - it's a pure phonetic fix. The trade-off is ambiguity: se lo doy could mean "I give it to him / her / you-formal / them". Spanish usually disambiguates by adding a + pronoun: se lo doy a ella, se lo doy a ustedes.

Worked examples

¿Me lo dices? (Are you telling me?)
Te la mando mañana. (I'll send it to you tomorrow.)
Se los compré ayer. (I bought them for him / her / them yesterday.)
Quiero dártelo. (I want to give it to you.)
Está leyéndolo. (He's reading it.)
No me lo digas. (Don't tell me.)

Common mistakes English speakers make

Leaving the pronoun after the verb out of English habit: veo lo instead of lo veo. Reversing the order: lo me da instead of me lo da. Forgetting the se lo rule: le lo doy instead of se lo doy. And dropping the written accent when attaching: damelo instead of dámelo.