How do I know when a verb takes essere or avere when forming the passato prossimo?

This can be a bit tricky! When forming the passato prossimo, we need an auxiliary verb (a helper verb - avere or essere) and then a past participle. Now, which auxiliary verb we use depends on whether the verb is transitive or intransitive. These words may be new to you but they quite simply explain whether or not a verb has an object. A transitive verb takes an object and an intransitive verb doesn't. A simple test is to add a noun after the verb and see if it fits! I eat an apple - 'eat' is the verb and it easily takes 'an apple' which is the object, therefore the verb is transitive.I go an apple - 'go' is the verb but the object 'an apple' doesn't fit with the verb, therefore the verb is transitive.Unfortunately, there are still a few exceptions! Camminare (to walk) takes avere even though it is an intransitive verb and diventare (to become) takes essere as an auxiliary.