[Jessica]

Originally Posted by irmamar

Direct Object: just for starting, you can distinguish it by asking the verb “what?”. Its not useful for every sentence, just for simple sentences (the best form would be to transform into passive voice or you can substitute the nouns by pronouns). Pronouns have already been mentioned.
Indirect Object: answers the question “¿a quién? / ¿para quién? I insist: it’s just for starting and for simple sentences.


A simple sentence as:

Voy a comprar ese vestido para mi sobrina (ese vestido –DO-; para mi sobrina –IO-)

¿Qué voy a comprar? – DO = ese vestido
¿a / para quién lo compro? – IO = para mi sobrina


Pronouns:

Voy a comprarlo para mi sobrina (lo = DO)
Se lo voy a comprar (se = IO –“le” is transformed here into “se” to avoid cacophony, but remember that “se” can have other functions)


Passive voice:

El vestido (DO is transformed into subject)
es comprado por mí
para mi sobrina (IO)


Position of pronouns:

IO + DO + V
Se lo voy a comprar

V + IO + DO
Voy a comprárselo


Duplication of the IO:

Se lo voy a comprar para mi sobrina.
Voy a decirles esto a mis compañeros (look the concordance pronoun and IO)


And about the sentence “vi el vestido”, remember that with some verbs some propositions are required in some occasions. For instance, with “ver” you must use the preposition “a” with persons and some animals you know (for instance a pet). Even if they use the preposition “a”, they’re still DO:

Vi a tu hermana la semana pasada (yo la vi, tu hermana fue vista por mí)
Vi a tu perro el día que se perdió.
Vi la oveja del vecino.
Vi el vestido en el escaparate.

good explanation