Modals

Could - Contractions and Reduced Pronunciation
... As already mentioned, most of the modals in combination with not form commonly used contractions can't, won't, etc ... Some of the modals also have contracted forms themselves The verb will is often contracted to 'll the same contraction may also represent shall ... also used for other cases of had as an auxiliary.) Certain of the modals generally have a weak pronunciation when they are not stressed or otherwise prominent for example ...

Could - Syntax
... A modal verb serves as an auxiliary to another verb, which appears in infinitive form (the bare infinitive, or the to-infinitive in the cases of ought and used as discussed above) ... The verb governed by the modal may be another auxiliary (necessarily one that can appear in infinitive form – this includes be and have, but not ... Hence a modal may introduce a chain (technically catena) of verb forms, in which the other auxiliaries express properties such as aspect and voice, as in He must have been given a ...

Linguistic Modality - Modal Categories - Epistemic Vs. Deontic Modality
... Epistemic modals are used to indicate the possibility or necessity of some piece of knowledge ... In the epistemic use, modals can be interpreted as indicating inference or some other process of reasoning involved in coming to the conclusion stated in the sentence containing the modal ... However, epistemic modals do not necessarily require inference, reasoning, or evidence ...

Could - Double Modals
... In formal standard English usage, more than one modal verb is not used consecutively, as modals are followed by an infinitive, which they themselves lack ... They can only be combined with non-modal constructions that have a modal function, such as have to, which in spite of its function is not a modal verb ... A greater variety of double modals appears in some regional dialects ...

English Modal Verbs
... The modal verbs of English are a small class of auxiliary verbs used mostly to express modality (properties such as possibility, obligation, etc.) ... The principal English modal verbs are can, could, may, might, must, shall, should, will and would ... Certain other verbs are sometimes, but not always, classed as modals these include ought, had better, and (in certain uses) dare and need ...