A distinção entre os dois verbos auxiliares é importante para a formação correcta dos tempos compostos e é também essencial para a concordância do particípio passado.
===The past participle===
The past participle is used in French as both an adjective and to form all the compound tenses of the language. When it is used as an adjective, it follows all the regular agreement rules of the language, but when it is used in compound tenses, it follows special agreement rules.
verbs form the participle by changing the -er ending to - é, -ir verbs by changing -ir to -i, and -re verbs by changing to -u. Therefore, the past participle of ''parler'', " to speak", is '' parlé''; for ''finir'', " to finish", ''fini'', and for vendre, " to sell", ''vendu''.
The rules of agreement for past participles differ for ''avoir'' verbs and ''être'' verbs ( see " Compound tense auxiliary verbs"). For ''avoir'' verbs, the past participle does not agree with the subject unless the direct object comes before the verb, either in the form of a pronoun or a relative clause using ''que''.
For the sixteen commonly used ''être'' verbs, the past participle always agrees with the subject. For reflexive verbs, the past participle generally agrees with the subject, unless there is a direct object to the reflexive verb.