The relation between the modal verbs 'can' and 'could' and the expression 'be able to' is discussed from both theoretical and practical perspectives. Theoretically, the problem of classification of the three verbs is discussed. The final position is that 'can' and 'could' should be treated as separate modals, rather than two forms of the same modal, due to the different usage of the two verbs. For the same reason, the phrase 'be able to' is taken to be a semi-modalized phrase.From the practical standpoint, the problems of Serbian ESL students mastering the three forms are presented. These include the students' choice of the verb form as well as their choice of the following infinitive. Several teaching methods to improve Serbian ESL student's use of these forms are suggested.