How do I know whether to use avoir or être in the passé composé?

The passé composé is the most common past tense in French and is used to talk about a completed action in the past. The passé composé is made up of two parts - the present tense of either avoir or être (the auxilliary) and a past participle (for example: j'ai mangé - I ate). Most of the time, avoir is used as the auxilliary verb but there are some exceptions that use être - these verbs are associated with movement and change and are sometimes know as the DR & MRS VANDERTRAMP verbs because they can be remembered like this:D - Devenir - to becomeR - Retourner - to return&M - Mourir - to die R - Revenir - to come back, returnS - Sortir - to exitV - Venir - to comeA - Arriver - to arriveN - Naître - to be bornD - Descendre - to go down; descendE - Entrer - to enterR - Rentrer - to returnT - Tomber - to fallR - Rester - to stayA - Aller - to goM - Monter - to go upP - Partir - to go; to leaveExample: Tu est parti - You leftAlso, all reflexive verbs (the 'se' verbs - se laver, s'appeler, se coucher, etc) always use être in the passé composé like the Dr and Mrs Vandertramp verbs, e.g. Je me suis baignée. Note: with any verb that uses être in the passé composé, the past participle must agree with the person - add an 'e' for feminine and an 's' for plural - e.g. Elles sont arrivées - they (fem) arrived