Published online by Cambridge University Press: 28 November 2008
The modal auxiliary should, when it is used to express ‘probability’, is generally considered as a weaker equivalent of must (see for instance Leech & Svartvik, 1975: 131 and Hornby, 1962: 223). Although this informal assumption contains a fair degree of truth, this paper will show first, that it requires qualification and secondly, that it gains strength once it has been qualified. In other words, there is more truth in the assumption than is suspected by its casual users.
The method I will use is to consider the compatibilities between must and should and their environment. Though the relevant kind of environment cannot be defined in a purely syntactic way, it can be accurately specified in simple semantic terms, many of which have syntactic correlates. Verbal contexts are generally available but situational contexts will occasionally have to be used. It will then appear that the diversity of situational contexts can be reduced to very few elementary relations.