In the French passé composé, most verbs take avoir, however there are some that take être. I sometimes have difficulties to remember which verbs are the ones that take être. This is not only important in the spoken language, but in the written language special rules on “concordance” apply. This concordance gives me special difficulty. The verbs that take être are usually action words or have to do with movement (also all the reflexitive verbs take être, but I won’t get into that, because that is pretty easy). In order to remember which verbs are the ones that take être, there are numerous mnemonic devices that can be used.
One that I know from many years ago, when I was starting to learn French is DR + MRS VANDERTRAMP, which stands for:
This however doesn’t include a verb like passer. So a better mnemonic device would be VANDERTRAMPP with two Ps at the end, with last P for Passer. There are also verbs derived from the above verbs, which also take être.
Of course to make this more complicated, this only applies when these verbs are used intransitively, that is that they have no direct object. When they have a direct object, then they usually require avoir.
Il est descendu. > notice that there is no direct object after descend
Il a descendu l’escalier. > notice the fact that there is a direct object (l’escalier) after
Look at this page for more details on this: