In the third person singular and plural (he, she, it = singular; they = plural) things can get a bit confusing at the start.

I give him the book. = Le regalo el libro. I give her the book. = Le regalo el libro. I give them the book. = Les regalo el libro.

The problem you will find is that sometimes le and les (the pronouns of the direct object) are used instead of lo and los. There is again a rule to this exception: Only if the object is a male person the pronoun of the direct object can be le or les. This substitution is not made in all of the Spanish speaking countries but it is generally considered correct.
If the object is a female person, a thing or an animal this substitution is not permitted.

The following summary should give you a good overview:

I see Pedro I see him   I see Maria I see her   I see the boys I see them   I see the girls I see them   I give Pedro a book I give him a book   I give Maria a book I give her a book   I give the boys a book I give them a book   I give the girls a book I give them a book

Le veo   correct Lo veo correct  Le veo   not correct La veo correct  Les veo   correct Los veo correct  Les veo   not correct Las veo correct  Le regalo un libro   correct Lo regalo un libro not correct  Le regalo un libro   correct Lo regalo un libro not correct  Les regalo un libro   correct Los regalo un libro not correct  Les regalo un libro    correct Los regalo un libro not correct

actually direct object (acusativo) rule of exception: Pedro = male   direct object pronoun (acusativo) no exception: Maria = female   direct object pronoun (acusativo) rule of exception: boys = male   direct object (acusativo) no exception: girls = female   indirect object (dativo) no substitution at all   indirect object (dativo) no substitution at all   indirect object (dativo) no substitution at all   indirect object (dativo) no substitution at all