At first sight, debere (`must, have to')  seems to have a structure similar to that of velle (`want'):
(118) debeo ire (`I must go') (119) volo ire (`I want to go')
(118) debeo ire (`I must go')
(119) volo ire (`I want to go')In other words, a two-place predicate with a Subject and a Complement. From a semantic point of view, it is less attractive to assume a similar structure for (118) and (119). Velle involves a relation of will between someone who wants something (Subject) and something that is wanted (Complement), the element of will being located in the former. Consequently, the Subject constituent is subject to semantic restrictions (must be human). Debere, on the other hand, is more adequately described as containing a relation of obligation between Subject and Complement where the element of obligation is not located in the former. The obligation may be due to external persons or entities. Debeo ire may be described as `it is obligatory, necessary, that I go', comparable to oportet me ire (`I ought to go'). If we assume the same semantic structure for debere as for oportet, the Subject I, semantically an argument of the embedded predication, fulfils the syntactic function Subject with the main predicate (so: Subject-to-Subject raising).  This leads to the representation as in figure 7.4.
Also in the case of posse in the sense `be possible'  a `Raising' analysis might be attractive. An example is (120):
(120) cum haec scribebam V Kalend., Pompeius iam Brundisium venisse poterat (`While I am writing this (25 February), it is possible that P. has already arrived in B.', Cic. Att. 8.9A.2)
(120) cum haec scribebam V Kalend., Pompeius iam Brundisium venisse poterat (`While I am writing this (25 February), it is possible that P. has already arrived in B.', Cic. Att. 8.9A.2)What is at stake here, is not so much Pompeius' capacities as the possibility that something has happened. 73a
Pinkster, Harm (1942-) , Latin Syntax and Semantics [info], xii, 320 p.: ill.; 24 cm. [word count] [Pinkster].