被 (bèi) makes a passive sentence: the person or thing that something happened to comes first, and the one who did it comes after 被. It is mostly used for things that went wrong or were unexpected, and it is a key grammar point of HSK 3.
Receiver + 被 + doer + verb + something else.
Start with the thing the action happened to, then 被, then who did it, then the verb. 我的手机被弟弟用坏了 tells the same story as 弟弟把我的手机用坏了, but it starts from my phone and what happened to it, not from my brother.
If you don't know or don't need to say who did it, put the verb straight after 被. To say "someone", use 被人.
As with 把, the verb in a 被 sentence can't stand alone. After it, add a result such as 坏 or 完, a direction such as 走 or 回家, or at least 了. 我的手机被弟弟用 is not a complete sentence.
English uses the passive far more often than Chinese. 被 is mostly for things that happened to someone or something and were bad, unwanted or unexpected. When it is obvious that the subject can't do the action itself, Chinese usually leaves 被 out: say 作业做完了, not 作业被做完了.
To say something didn't happen, put 没有 before 被, not before the verb, and leave out 了. To warn someone, put 别 before 被. To ask who did it, put 谁 after 被.
In everyday speech, 让 can take the place of 被, but then you must say who did it.
Practise this grammar point on paper: 20 exercises using only HSK 3 words, with an answer key on a separate page. Free to print and copy for your class.
Every example on this page uses only words up to HSK 3. Paste your own sentences into the level checker to see which words, if any, are above your level.