How French works
In a compound tense like the perfect, the past participle of avoir verbs only takes an agreement if there is a direct object coming before the participle. This is called a preceding direct object agreement and is generally only used in more formal writing or speech. A preceding direct object can take several forms:
The relative pronoun que and the noun it replaces:
Voilà la veste que j'ai achetée
Here's the jacket I bought
Les décisions qu'il a prises étaient bonnes
The decisions (that) he made were good
Certain question words: lequel, laquelle etc., combien + noun, quel, quelle etc. + noun:
Laquelle des deux maisons as-tu préférée?
Which of the two houses did you like best?
Combien de feuilles de papier a-t-il prises?
How many sheets of paper did he take?
Quelles raisons a-t-il données?
What reasons did he give?
The personal pronouns le, la and les, and me, te, nous, vous where these act as a direct object :
La lettre? Je l'ai envoyée la semaine dernière
The letter? I sent it last week
- Avez-vous vu mes lunettes? - Oui, je les ai vues sur la table
- Have you seen my glasses? - Yes, I saw them on the table
As the pronouns me, te, nous, vous can be direct or indirect , it is easy to confuse the two and get the wrong p.d.o. agreement:
Elle nous a donné un conseil
[and not *Elle nous a donnés un conseil]
She gave us a piece of advice
Le concert nous a beaucoup plu
[and not *Le concert nous a beaucoup plus]
We very much enjoyed the concert
Here donné and plu do not take an agreement because they are both indirect (i.e. donner qch à qn - to give sb sth, plaire à qn - to please sb).