受事 + 动词 + 了【shòushì + dòngcí + le】

The commonest passive has no marker at all. The thing acted on goes first as the topic, and the verb with its complement follows. Who did it either doesn't matter or is obvious.

This works when the first noun obviously can't be the doer: tickets don't sell anything, homework doesn't do anything. 被 is reserved for something that happened to someone, usually unwelcome.

• × 作业被做完了 → 作业做完了
• × 饭被做好了 → 饭做好了

The verb needs a complement or 了

As with 把 and 被, the verb can't be left bare at the end.

• × 信写 → 信写好了

Naming who did it

A doer can go between the topic and the verb, with no marker.

When it's ambiguous, use 被

If the first noun could be the doer, a sentence with no marker reads as active. 鸡吃了 can mean "the chicken has eaten". Add 被 to make it passive.