How do you form the past conditional tense in French?

In French, the past conditional tense (conditionnel passé) is used to express a hypothetical action that would have occurred in the past if certain conditions had been met. To form the past conditional tense, you need to use the conditional form of the auxiliary verb "avoir" or "être" followed by the past participle of the main verb. Here's how it's formed:

1- Using the conditional of the auxiliary verb:

The conditional form of "avoir" or "être" is used as the auxiliary verb, depending on whether the main verb is transitive or intransitive.

The conditional forms of "avoir" are: 

• J'aurais

• tu aurais

• il/elle/on aurait

• nous aurions

• vous auriez

• ils/elles auraient.

The conditional forms of "être" are: 

• je serais

• tu serais

• il/elle/on serait

• nous serions

• vous seriez

• ils/elles seraient.

2- Adding the past participle of the main verb:

• The past participle of the main verb is added after the conditional form of the auxiliary verb.

• The past participle agrees in gender and number with the subject when the auxiliary verb is "être."

• The past participle remains unchanged when the auxiliary verb is "avoir."

Examples:

• J'aurais fini. (I would have finished.)

• Tu serais venu(e). (You would have come.)

• Elle aurait parlé. (She would have spoken.)

• Nous serions partis(es). (We would have left.)

• Vous auriez lu. (You would have read.)

• Ils seraient arrivés. (They would have arrived.)

In summary, the past conditional tense in French is formed by using the conditional form of the auxiliary verb "avoir" or "être" followed by the past participle of the main verb. It is used to express hypothetical actions or events that would have occurred in the past under certain conditions.