Do modal verbs (can, may, must, etc.) in any way affect the semantic roles of the arguments of the verbs that they govern?
For example, consider the simple sentence:
He plays basketball.
Here, if I am not mistaken, 'He' (the subject of 'plays') has the semantic role of agent (since 'He' is doing the playing), and 'basketball' (the direct object of 'plays') has the semantic role of patient (since 'He' is doing something to 'basketball', namely playing it).
However, if one adds a modal verb to the sentence, it seems to me that the semantic roles change. For example:
He must play basketball.
The two possible interpretations of this sentence are: (a) it is essential that he play basketball (deontic); or (b) from the available evidence, it is likely that he plays basketball (epistemic).
Anyway, using the above, 'modalised' sentence, what exactly are the semantic roles of subject and direct object? To me, they seem very different from the 'non-modalised' sentence, since (at least as it seems to me) the agent ('He') no longer 'carries out' the action of playing basketball; instead, the agent must play basketball. Similarly, the patient ('basketball') does not appear to be affected by the agent in the above sentence.
Or am I wrong? Do modal verbs not modify the semantic roles of the subject and object, only the sentence's modality?
I apologize if this question is not written in the clearest terms. My question is merely whether or not modal verbs impact upon the semantic roles of a sentence.
(This question probably belongs in the English Language StackExchange; however, I have opted not to post it there, since such questions as those that relate to e.g. semantic roles tend to receive answers characterised by a lack of the necessary linguistic knowledge to answer those kinds of questions. But I know that these kinds of questions often tend not to be received too kindly on the Linguistics Stack Exchange, since they are 'not 'linguistic enough', so to speak. This question is, to put it short, neither 'linguistic' enough to be here, nor 'grammatical' enough to be on English Language StackExchange. I post this question here knowing that it is likely that it will be flagged for 'not belonging on Linguistics StackExchange'.)
