Once again a summary of the rules: Reflexive verbs are conjugated with essere, up to here nothing really complicated. The situation changes if there is a modal verb because if there is modal verb there is an infinitive as well and so you can put the reflexive pronoun in front of the first conjugated verb or you can add to the infinitive. If the reflexive pronoun is in front of the first conjugated verb the verb is conjugated with essere. But if the reflexive pronoun is added to the infinitiv, the auxiliary verb to be used, essere or avere, depends on the main verb (the infinitive in this case). If the main verb is conjugated normally with essere, it is conjugated with essere otherwise with avere.