The modal verbs: moći and umjeti »can«, znati »to know«/»can«, htjeti »will«, voljeti »to prefer«, smjeti »to dare«/»may«, morati »must«, trebati and valjati »shall«, imati »to have to«/»must«, and dati »to let«, constitute, with regard to their aspectual properties, a special group within the framework of the Serbo- Croatian verb. They have two characteristic traits: (1) Most of them tend to double their present-tense forms, e. g. mogu : mog- nem, for the needs — as generally taught — of the aspectual opposition and also have two aorist forms, e. g. htjeh : htjedoh; moreover, some of them form compound perfective pairs, e. g. uzmoći, which are used, if at all, in the present tense only as a rule. (2) Otherwise, in other syntactical situations, they make use of the same forms. Both facts operate upon the aspectual value of the verbs in question, all of which — except the ostensibly perfective dati — are usually considered as imperfective. Yet the analysis of various temporal, modal, and nominal uses of their forms indicates that these verbs n e u tra liz e the aspectual opposition »imperfective« : »perfective« in certain syntactical situations, so that they can cover, if not express, both aspectual meanings. In other words, they resemble the so-called »verbs of two aspects«, which in reality are aspectually neutral. The modal verbs distinguish three kinds of aspectual forms: (a) 'the present tense mogu, etc., the imperfect, and the present gerund, which are, in principle, imperfective; (b) the present tense mognem, etc., which is used in situations which admit of perfective verbs only, although its meaning is not necessarily perfective; (c) the rest of the forms, which are aspectually neutral. The first two ranges of forms are likely to serve not so much for different aspectual meanings as for different aspectual functions. It is quite probable that these verbs carry their aspectual neutrality from their origin and have not succeeded, in their morphological development, to build up a complete range of aspectual forms, so that most of their forms are used in both aspectual situations. The same could be shown in the domain of the Slavic verb in general;
