What is the Preceding Direct Object rule?

I distinctly remember my A-Level French teacher repeatedly highlighting mistakes such as the one below in my essays:La fille que j'ai vu à côté du restaurant, elle pleurait.In French when we use a sentence in the Passé Composé that contains a 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.We do this we both avoir as the auxilary verb as well as être, and including reflexive verbs.The endings are the same ones we would add to DOP used alongside être in phrases such as 'Elle est contente'. They are as follows:Masc. Singular- no changeMasc. Plural - add 's'Fem. Singular - add 'e'Fem. Plural - add 'es'So, a correct version of the first example would b :La fille que j'ai vue à cöté du restaurant, elle pleuraitHere, the femenine singular 'e' ending has been added to the past participle 'vu' to adhere to the preceding direct object rule as the pronoun arrives before the past participle. My teacher used to say that if you are referring BACK to an object, always check agreements!