When do you have to make past participles agree if you are using avoir?

The basic rule is that you don’t have to make the past participle agree with the object (the noun) in the passé composé: ‘I picked the flowers’ is ‘j’ai cueilli les fleurs’ even though ‘fleurs’ is feminine plural. But if the object precedes the verb, you have to make the past participle agree. Let’s say you wanted to say ‘the flowers that I picked…’. Here the object – the flowers – precedes the verb – picked; that means that you would have to make the participle agree in French. That would be: ‘les fleurs que j’ai cueillies…’. It’s the same if you are using a direct object pronoun (that’s where you put le, la, or les before the verb instead of repeating the noun). If someone said to you, ‘Est-ce que vous avez vu la maison?’, you would reply, ‘Non, je ne l’ai pas vue’. Here the l’ stands in for ‘la maison’ and it precedes the verb which means that you have to make the participle agree (vue).