Modal verbs: modal verbs overview
DOVERE
to have to		 POTERE
to be able to		 VOLERE
to want/ wish		 SAPERE
to know how to

Modal verbs describe the link between the subject and the main verb (which is in the infinitive). It cretaes a mood for the verb which follows.

The position of the pronoun is optional- it can precede the modal verb or be attatched to the end of the infinitive, e.g:
ti voglio vedere/ voglio vederti
la posso aiutare/ posso aiutarla
le devo telefonare/ devo telefonarle
ci devo andare/ devo andarci (i have to go there)