How do you form the passé composé with être?

There are fourteen main verbs which need to use être in the passé composé (past tense) and these can be remembered in one of two ways: either by MR RED VAN TRAMPS or MRS VAN DER TRAMP. These verbs are (if I use MR RED VAN TRAMPS) Monter, Rentrer, Retourner, Entrer, Descendre, Venir, Aller, Naitre, Tomber, Rester, Arriver, Mourir, Partir and Sortir. Some say that there are other verbs that need to use être instead of avoir in the past like devenir, revenir etc. but these have the verb 'venir' in them and so we know that we have to use être as venir is one of the fourteen main verbs. Most of these verbs describe a change of state or motion, a transformation and this helps remind us to use être. Être is formed in a similar way to avoir:JE suisTU es IL/ELLE/ON est NOUS sommesVOUS êtesILS/ELLES sontIt is also important to note that these conjugated verbs are variable and therefore need to be changed depending on who has done the action. If I am a girl, an extra 'e' must be added, if I am talking about a group of people, an extra 's' must be added and if I am talking about a group of girls, I must add an extra 'es'.Some examples of this would be:I went to the beach je suis allé(e) à la plage We left together Nous sommes partis ensembleThey (girls) were born at the same timeElles sont nées à la fois