What is the difference between using the 'etre' form and the 'avoir' form when conjugating the past tense?

The majority of verbs take the 'avoir' form when conjugated into the past tense. Such as 'J'ai mangé', 'J'ai fait', 'J'ai téléphoné'. However, certain verbs take the 'etre' form. It is sometimes difficult to be sure as to whether a verb will take the 'etre' when conjugated into the past, but there are a couple of rules that help us to identify them. Firstly, ALL reflexive verbs take 'etre', such as, 'Je me suis lavé', 'Je me suis couché'. Secondly, verbs to do with movement tend to take 'etre'. To help us remember some of these, we use a handy acronym: MRS VAN DE TRAMP. Each letter corresponds with a verb that takes 'etre' in the past: Mourir, Retourner, Sortir, Venir, Aller, Naitre, Descendre, Entrer, Tomber, Arriver, Mourir, Partir. The one that's most important to remember is 'Aller', because you'll be using that one a lot. So, if you were to say 'I went', you'd say 'Je suis allé'. The third ESSENTIAL rule about etre verbs, is that the past participle must always agree with the subject. This is not the case with avoir verbs. So, if you were a girl you'd stick an extra 'e' on the end of 'allé', making it 'allée'. Equally, if you are talking about more than one person, you'd need to put an 's' on the end. So, if a group of girls were to say 'we went', it would look like this: 'nous sommes allées'.