modality

must (deontic)

strong Sp/H obligation; speaker imposes rule; inner compulsion

need (negative contexts)

Sp/H; “no need” according to speaker; rhetorical questions

have to

strong non‑Sp/H obligation from rules, circumstances, third party

have got to

non‑Sp/H; informal; single occasions only

need to

non‑Sp/H; obligation for a purpose

be to

non‑Sp/H; official orders, arrangements, destiny

should

weak obligation; moral advice; Sp/H irrelevant

ought to

weak obligation; moral advice; Sp/H irrelevant

must not/ mustn’t

strong Sp/H prohibition; internal negation

not be to

strong non‑Sp/H prohibition; external authority

should not/ shouldn’t

weak prohibition; moral advice; Sp/H irrelevant

ought not to

weak prohibition; moral advice; Sp/H irrelevant

not have to

non‑Sp/H; absence of obligation from external source

not need to

non‑Sp/H; absence of external need

need not/ needn’t

Sp/H; speaker says “no need”

shall

Sp/H volition; speaker’s intention, promise, insistence

will (volition)

non‑Sp/H; subject’s willingness, habits, promises

would (volition)

non‑Sp/H; polite, hypothetical, past volition

may (permission)

Sp/H permission; formal

can (permission - modern use)

Sp/H; informal permission

can (rules)

non‑Sp/H; general permission from rules

could/ might (permission)

non‑Sp/H; polite, tentative permission

can (ability)

non‑Sp/H; intrinsic ability

could (ability)

non‑Sp/H; past or hypothetical ability

be able to / manage to

non‑Sp/H; specific ability

must (epistemic)

non‑Sp/H; strong logical necessity

may/ might/ could (epistemic)

non‑Sp/H; possibility

should (epistemic)

non‑Sp/H; probability

cannot/ could not (epistemic)

non‑Sp/H; logical impossibility