What does PDO stand for, and what does it mean?

This stands for Preceeding Direct Object, and is a grammatical term. Basically, you must always agree a past participle if there is a direct object that comes before it in the sentence. For example, 'il avait une poupée. Il l'a aimée beaucoup' meaning ' he had a doll. He loved it very much'. Ask yourself, 'he loved what?' and if the object comes before - here, 'une poupée' - then you must make the verb in the past tense agree with this object.

Remember to agree with gender and number, for example 'As-tu envoyé les lettres? Oui, je les a envoyées ce matin' because 'lettres' is feminine plural, so the past participle envoyer (to send) must agree with them.

As-tu aimé les films? Non, je ne les a aimés du tout. Did you like the films? No, I did not like them at all. Here, aimé becomes plural, because film is masculine plural.