蛋糕吃光了: the passive without 被
【五40】被动句 / special sentence patterns
Tone colorsā tone 1á tone 2ǎ tone 3à tone 4a neutral
In the notional passive (意念被动句) the thing affected is the subject, followed by a verb with a result complement (and often 了), with no 被 and no agent: 蛋糕吃光了 ("the cake is all eaten"), 车票卖完了, 作业写好了, 衣服穿破了.
It reports a state of affairs without asking who is responsible: where English says "the cake is gone", "the tickets are sold out" or "it's booked", Chinese simply keeps the thing as the subject. The verb is never bare: it needs a complement (完, 光, 好, 破, 清楚…) or 了.
To name the doer, switch pattern: 被 (蛋糕被我吃光了), which spotlights the agent or often hints at a mishap, or 把 (我把蛋糕吃光了). Compared with the 被 / 叫 / 让 passive, the sentence without 被 is neutral, and very common in speech and in writing.
Examples
蛋糕吃光了。
Dàngāo chī guāng le.The cake is all gone.
车票卖完了。
Chēpiào mài wán le.The tickets are sold out.
作业写好了，可以出去玩了。
Zuòyè xiě hǎo le, kěyǐ chūqù wán le.The homework is done, so I can go out and play.
包间已经订好了。
Bāojiān yǐjīng dìng hǎo le.The private room is already booked.
这件衣服穿破了，不能再穿了。
Zhè jiàn yīfu chuān pò le, bù néng zài chuān le.This garment has worn through and can't be worn any more.
窗户擦干净了，桌子也摆好了。
Chuānghu cā gānjìng le, zhuōzi yě bǎi hǎo le.The windows are clean and the table is set.
Common mistakes
✗ 蛋糕吃 → ✓ 蛋糕吃光了 (the verb needs a complement or 了).
✗ 蛋糕吃光了我 → ✓ 蛋糕被我吃光了 (to name the agent you need 被 or 把: 我把蛋糕吃光了).