Modal

Current Form: Can
Past Form: Could
Other Modals:
- may, might, shall, should, will, would, must, (have to), (had to), ought to

Modal auxiliary verbs are always finite verbs which indicate present tense and form in the past.
Modals are always tensed verbs (denoted as [+T]).
Modals occupy the first slot in verb phrases (VP).
Modals can be combined with lexical verbs (negative structure).
They can appear before the subject (interrogative structure).
Unlike lexical verbs, modals do not have all five forms.
Past forms of modals include: used (to), should, would, could, might. - Example: - "Hari used to smoke" (indicates past habits).

Modals express various concepts such as: - Ability, possibility, probability, permission, wishes, purpose, futurity, prediction, volition, advice, suggestion, warning, threats, promise, requests, obligations, inevitability, etc.
Examples of modals summarizing their meanings: - Ability: "I can swim" - Permission: "May I go?" - Suggestion: "You should see a doctor."

Past forms indicate a past time significance: - Example: "I told Hari I should come the next day" (future in the past). - "You should rest now" (present time). - "You should rest tomorrow" (future).
Specific forms: - Could: "Hari could swim in his childhood" (past ability). - Might: "It might rain today" (present/future weak possibility).

Examples showing equivalent meanings from modals: - Can - Could - Will - Would - Shall - Should - May - Might
Implications of past forms are perceived as weaker or less positive.

When linking modals with perfect aspect, use: - Could have, would have, should have, might have.
Example: - "Yesterday it might have rained" (if rain occurred in the past). - "K.L. Rahul could have scored a century" (unfulfilled potential).

Ability/Capability: - Examples: - "Hari can speak French." - "Can you lift this box?"
General/Theoretical Possibility: - "Fire can burn."
Informal Permission: - Example: "Can I come in?"
Request: - Example: "Can you lend me your pen?"
Offering Help: - Example: "Can I carry your books?"
Use with Sense Verbs: - Example: "I can hear music."
Idiomatic Expressions: - Example: "I cannot but laugh." - "I cannot help but enjoy my time."

Factual Possibility: - Example: "It may rain today."
Formal Permission: - Example: "May I come in?"
Purpose: - Example: "Mohit works hard so that he may obtain the highest marks."
Wish: - Example: "May you live long!"
Idiomatic Expression: - Example: "She may well go alone."

Modals serve crucial roles in English language by indicating various nuances in meaning pertaining to ability, permission, obligation, possibility, and more. Understanding their correct application is vital for effective communication in English.