可: the emphatic adverb (可漂亮了, 可不能…)
【五03】【五08】【五58】副词 / adverbs (emphasis)
Tone colorsā tone 1á tone 2ǎ tone 3à tone 4a neutral
The adverb 可 is a spoken amplifier: it puts weight behind what you say, like "really" or "I can tell you" in English. It shows up in three patterns: 可 + adjective + 了, 可 + 不 / 没 + verb and 你可 + 得 / 要 / 不能 + verb.
With an adjective, 可 + Adj + 了 is an exclamation: 她可漂亮了 ("she is really pretty"). The final 了 is needed, and 可 replaces 很 instead of adding to it. Before a negation, 可 + 不 / 没 + verb states firmly that something is not or will not be the case: 我可记不住这么多生词 ("there's no way I'll remember so many new words").
With 你可得 / 要 / 不能 + verb, 可 becomes a strong warning or reminder: 你可得注意身体 ("do take care of your health"), 你可不能让大家失望. And 你可来了! means "you're here at last!".
Don't mix it up with the conjunction 可 / 可是 ("but"), which links two clauses: this 可 follows the subject and stands before the adjective or verb. Next to 很, 非常 or 太…了, it is the most colloquial and expressive choice, so keep it for conversation.
Examples
她唱歌可好听了！
Tā chànggē kě hǎotīng le!She sings really beautifully!
这家店的烤鸭可好吃了。
Zhè jiā diàn de kǎoyā kě hǎochī le.The roast duck at this place is really good.
我可记不住这么多生词。
Wǒ kě jì bu zhù zhème duō shēngcí.There's no way I'll remember so many new words.
这件事你可不能告诉别人。
Zhè jiàn shì nǐ kě bù néng gàosu biérén.You really mustn't tell anyone about this.
你可得注意身体，别太累了。
Nǐ kě děi zhùyì shēntǐ, bié tài lèi le.Do look after your health and don't overdo it.
你可来了，大家都等了你半天了！
Nǐ kě lái le, dàjiā dōu děng le nǐ bàntiān le!You're finally here, we've all been waiting for ages!
Common mistakes
✗ 她可漂亮 → ✓ 她可漂亮了 (the exclamatory 可 comes with 了).
✗ 你可注意身体 → ✓ 你可得注意身体 (a warning needs a modal such as 得, 要 or 不能).
✗ 她可很漂亮了 → ✓ 她可漂亮了 (可 takes the place of 很, it does not stack with it).