There are two types of object pronouns: direct and indirect.
|te||you (singular - informal)|
|lo||him/it/you (singular - formal)|
|la||her/it/you (singular - formal)|
|os||you (plural - informal)|
|los/las||them/you (plural - formal)|
Indirect object pronouns are used to replace a noun which is not the direct object of the verb, eg I gave it to them.
|te||(to) you (singular - informal)|
|le||(to) him/it/you (singular - formal)|
|os||(to) you (plural - informal)|
|les||(to) them/you (plural - formal)|
Direct and indirect object pronouns both come in front of the verb. For example:
Lo hice. (I did it.)
Los compré. (I bought them.)
Le escribí. (I wrote to him.)
Los profesores nos dan muchos deberes. (The teachers give a lot of homework to us.)
They can also go at the end of an infinitive, eg voy a comprarlo (I am going to buy it), tenemos que darles un regalo (we have to give a present to them).
Replace the nouns in bold with the correct object pronoun.