[Grammar] Modals, degrees of certainty

[Venus.jam]

hi

In present negative time we can say for example, "he could not/cannot be sick". But, in present positive time we can only use "he could be sick". I wonder if we can also use "he can be sick" as well and if not what is the reason?

 

[Venus.jam]

I see. So, does this mean that grammar of all languages including English is not static and we may see some other ways through which sentences are produced? I mean can we say that we cannot equate grammar with rules due to the evolving nature of language?

 

[GoesStation]

Speakers of all languages at all registers follow rules, some more rigidly than others. All living languages evolve. Some aspects of languages change more readily than others. I'm not a linguist but I think it's clear that vocabulary changes faster than pronunciation, for example.