Same answer: one marked right, the other marked wrong

Bonjour Donna, 

I could only find the first example as a quiz question in this lesson - which is correct because it deals with what we call,  'direct object pronouns' - le/la/les

Your second example uses 'lui' which is an indirect object pronoun, because the verb 'apprendre quelque chose à quelqu'un' ( to teach someone something)  here has an à after it 

This is the lesson that deals with indirect object pronouns -

Using lui/leur = him or her/them (French Indirect Object Pronouns)

I hope that this explains the difference but I would be interested to know where the second example came from.