Answer:
1) Pedro nunca nos llama.
2) Pedro siempre la llama por teléfono celular.
3) Él no lo sabe, pero la podemos escuchar.
4) Pedro le compró unas flores.
5) Su novia prefiere las rosas rojas y no las que él compró.
6) Pedro compró más flores. Esta vez se las compró rojas. Y ahora su novia está feliz.
Explanation:
1) Peter never calls us.
2) Pedro always calls her by cell phone.
3) He doesn't know it, but we can hear it.
4) Pedro bought him some flowers.
5) His girlfriend prefers red roses and not the ones he bought.
6) Pedro bought more flowers. This time he bought them red. And now his girlfriend is happy.
The pronouns of the indirect object replace a word or phrase, which in the sentence fulfills that function (indirect object).
The pronoun of the indirect object is placed before the verb, when it is conjugated.