Almost all verbs will use 'avoir' when forming a compound tense - the perfect tense for example. This can be seen in the following example - "j'ai visité" (I visited). Here, the 1st person, singular form of 'avoir', 'ai' (I have), is used as the auxiliary, while the past participle of the verb 'visiter' (to visit) - 'visité' - is also used to form the perfect tense. There are two main types/categories of verbs which require 'être', these are reflexive verbs - where the verb refers back to the subject - and verbs in the MRSVANDERTRAMP sequence. An example of a reflexive verb would be 'se laver' (to wash oneself), and the perfect tense of this would be 'je me suis lavé(e)' (I(have) washed myself). An example of a 'MRSVANDERTRAMP' verb is 'venir' (to come), which would be formed as 'je suis venu(e)' (I came/have come) in the perfect tense.