What is the Preceding Direct Object (PDO) rule?

In French when we use a sentence in the Passé Composé that contains a direct object or direct object pronoun BEFORE the past participle, we must alter the ending of the past participle in accordance with the (i)gender and (ii)quantity of the direct object.

This situation arrises with AVOIR as the auxiliary verb and with REFLEXIVE VERBS using ETRE when the subject is the direct object of the verb.

The endings we add are as follows;

Masc. Singular- no change

Masc. Plural - add 's'

Fem. Singular - add 'e'

Fem. Plural - add 'es'

For example;

J'ai connu les filles il y a un mois - I met the girls last month

Here, there is no agreement with the feminine plural direct object and the past participle of connaitre as the direct object arrives after the past participle.

However;

Je les ai connues il y a un mois - I met them (the girls) last month

Here, the femenine plural 'es' ending has been added to the past participle 'connu' to adhere to the preceding direct object rule as the pronoun arrives before the past participle.