Contents introduction chapter I grammatical meaning, grammatical form

4. The principal English modal verbs and their definition.
Can and could. The modal verb can expresses possibility in either a dynamic, deontic or epistemic sense, that is, in terms of innate ability, permissibility, or possible circumstance.⁹ For example:
I can speak English means "I am able to speak English" or "I know how to speak English". You can smoke here means "you may (are permitted to) smoke here" (in formal English may or might is sometimes considered more correct than can or could in these senses). There can be strong rivalry between sibling's means that such rivalry is possible. The preterit form could is used as the past tense or conditional form of can in the above meanings (see Past forms above). It is also used to express possible circumstance: We could be in trouble here. It is preferable to use could, may or might rather than can when expressing possible circumstance in a particular situation (as opposed to the general case, as in the "rivalry"¹⁰ example above, where can or may is used). Both can and could can be used to make requests: Can/could you pass me the cheese? means "Please pass me the cheese" (where could indicates greater politeness). It is common to use can with verbs of perception such as see, hear, etc., as in I can see a tree. Aspectual distinctions can be made, such as I could see it (ongoing state) vs. I saw it (event).
The use of could with the perfect infinitive express past ability or possibility, either in some counterfactual circumstance (I could have told him if I had seen him), or in some real circumstance where the act in question was not in fact realized: I could have told him yesterday (but in fact I didn't). The use of can with the perfect infinitive, can have..., is a rarer alternative to may have... The negation of can is the single word cannot, only occasionally written separately as can not. Though cannot is preferred (as can not is potentially ambiguous), its irregularity (all other unconstructed verbal negations use at least two words) sometimes causes those unfamiliar with the nuances of English spelling to use the separated form. Its
⁹Comrie, Bernard, Tense, Cambridge Univ. Press, 1985, p. 47.
¹⁰ Oxford Practice Grammar (Advanced), George Yule, Oxford University, p. 40.
contracted form is can't (pronounced/ka:nt/in RP and some other dialects). The
negation of could is the regular could not, contracted to couldn't. The negative forms reverse the meaning of the modal (to express inability, impermissibility or impossibility)¹¹. This differs from the case with may or might used to express possibility: it can't be true has a different meaning than it may not be true.
Thus can't (or cannot) is often used to express disbelief in the possibility of something, as must expresses belief in the certainty of something. When the circumstance in question refers to the past, the form with the perfect infinitive is used: he can't (cannot) have done it means "I believe it impossible that he did it" (compare he must have done it). Occasionally not is applied to the infinitive rather than to the modal (stress would then be applied to make the meaning clear): I could not do that, but I'm going to do it anyway.