In front of the conjugated verb
English has one habit: the object pronoun follows the verb — I see it, I call you. Spanish object pronouns (lo, la, te, me, le, nos ...) never do that. They either stand in front of the conjugated verb as a separate word, or they are glued onto an infinitive as part of the same word. The verb form decides which one you get.
Lo veo.
I see it.
Te llamo.
I'm calling you.
No le digo nada.
I'm not telling him / her anything.
Attached to the infinitive
The pronoun of an infinitive is attached to its end. Verb and pronoun form one word, with no hyphen and no space. This is the only option when the infinitive stands alone, for example after a preposition.
Vengo para verte.
I'm coming to see you.
Gracias por llamarme.
Thanks for calling me.
Verb + infinitive: two options
After querer, necesitar or ir a you may put the pronoun in front of the conjugated verb or attach it to the infinitive. Both are correct and mean the same thing; the front position is a little more common in everyday speech. The negative no goes in front of the conjugated verb in both cases: No lo quiero ver, No quiero verlo.
In front of the conjugated verb
Lo quiero ver.
I want to see it.
Te voy a llamar.
I'm going to call you.
Las voy a comprar.
I'm going to buy them.
Attached to the infinitive
Quiero verlo.
I want to see it.
Voy a llamarte.
I'm going to call you.
Voy a comprarlas.
I'm going to buy them.
The same holds for the gerund: the pronoun goes in front of the auxiliary verb or is attached.
