What is the difference between avoir and etre in the passé composé?

Avoir (to have) and être (to be) are both auxiliary verbs used in the perfect tense in French. The majority of words take avoir in the passé composé, e.g. 'I played' becomes j'ai joué. All reflexive verbs in French take être - these are verbs that you do to yourself (e.g. je me suis habillé). There are very few other verbs in French that take être in the perfect tense, but it is important to learn them. Luckily there is a simple mnemonic to remember these verbs - just remember to ask Mrs Vandertramp!Monter (to go up)Retourner (to return)Sortir (to go out)Venir (to come) - also includes words using venir, such as revenir and devenirAller (to go)Naître (to be born)Descendre (to go down)Entrer (to enter)Rentrer (to return/to go home)Tomber (to fall)Rester (to stay/to remain)Arriver (to arrive)Mourir (to die)Partir (to leave)These are formed just like avoir verbs - with the auxiliary and past participle. However, a key difference is that verbs formed with être in the perfect tense have agreements with the past participle. For example, if we were to say 'she went out' in French, we would need to add an 'e' to the past participle - elle est sortie. If we were to say 'they left' (mixture of boys and girls), we would need to add an 's' to the past participle - ils sont partis.