Question
First i would like to have an explanation of how this grammar works:
Tu as acheté un cadeau pour votre amie ?
Answer
The answer is supposed to be:
Oui, je l'ai acheté.
I chose oui, je l'ai achetee, which is incorrect.
Please explain how my answer is incorrect.
Where my confusion happens
Tu as acheté un cadeau pour votre amie ?
The answer to that is supposed to be:
Oui, je l'ai acheté
How is that so?
I suppose you heard about the rule of past participle agreement with the direct object complement, if you're asking such a question, so I'm not going to explain it in details. Let's just see why it's not -ée:
Oui je l'ai acheté. / Yes, I bought it.
... l' is for it and it refers to le cadeau, which is masculine. You don't want to make the past participle agree with amie here.
Note that with the question Tu as acheté un cadeau pour votre amie ?, since it's un cadeau and not le cadeau, the answer should better be: Oui, j'en ai acheté un (I bought one).
To complete the @Simon Déchamps's answer
When you do not know to what it refers you can use this trick :
The sentence :
Oui je l'ai acheté/achetée (is "l" masculine or feminine ?)
Ask yourself :
Oui j'ai acheté quoi ? J'ai acheté le cadeau
You now know what is the object. In this case it is a masculine word then you do not add the "e"
Oui je l'ai acheté !
Accord avec l'auxiliaire avoir
Le participe passé ne s'accorde jamais avec le sujet lorsqu'il est employé avec l'auxiliaire avoir:
for example : elles ont mangé au restaurant du coin.
It means that when you have a phrase with the verb 'avoir' the past participle never agrees with the subject and it's always "e" and NEVER "ée"
For example : j'ai acheté une fleur. J'ai acheté un coffre.
