可grammar point · tier 1 · 可 — bookish modal: can be / is worth -ing (可 + bare verb)
· kě
Sits directly before a single bare verb in written register: the action is possible or worth doing, and the verb is read in the passive-potential — can be done, is -able.
Sits directly before a single bare verb in written register
框 · Frame
[subj] 可 [bare monosyllabic verb] (written register; no 以)
觸 · Trigger
In a written sentence you want to say a verb is doable or worth doing, in one tight syllable.
序 · The move
1Set 可 directly before a single bare verb, with no 以 after it.Is this written or formal register? In speech this reads as clipped — 可以 / 能 is the spoken form.
2Read the verb passively-potentially: it can be done, it is -able.Does 可V mean 'V-able' (可信 = believable, 可見 = visible)? If you mean a person is permitted, that is 可以.
3Keep the verb bare and usually monosyllabic; do not append an object the way 可以 would.Could you swap in 可以 + verb phrase and stay grammatical in speech? Then the bookish 可 is the terse twin of that.
例 · Examples
1他不但not only可can (be); is able to be (written)理解understand老師上課的內容content，還also; on top of that能can (has the capacity to)說出state, voice重點the key points。
Not only can he understand the content of the teacher's lesson, he can also state the key points.
可2 fuses the yes straight onto one bare verb and reads it passively-potentially (可理解 = able to be understood), bookish; 可以 hands the yes forward through 以 to a full verb phrase a speaker says aloud (可以理解 = can understand). No 以, terse, -able reading = 可2; 以 present, spoken, full predicate = 可以.
可2 says the verb is doable in the abstract and worth doing, in written register; 能 says a subject has the capacity or conditions to act, in any register. 可信: worth trusting; 我能信你: I am able to trust you.
他可以理解的內容很多。 (spoken 可以 dropped into a clipped written slot where bare 可 fits) → 可理解的內容很多。 (bare 可 + verb in written register)
這件事可相信他。 (giving 可 an object as if it were 可以) → 這件事可信。 (可 + bare verb, read -able; the verb takes no object)
他可去。 used in casual speech for permission → 他可以去。 (spoken permission is 可以; bare 可 stays in written register)
English 'can' is one word across speech and writing, so learners use 可以 everywhere and never reach for the terse written 可; conversely the passive '-able' sense (readable, believable) is one English suffix, hiding that Chinese builds it with this 可.