The ba sentence and the bei passive 把字句与被字句

Learners think 把 sentences and plain SVO mean the same and can be swapped freely. WRONG: 把 specifically fronts a DEFINITE object being disposed of or completed, and it REQUIRES a result/complement after the verb. ✗ 我把一本书看了. ✓ 我看了一本书 (plain SVO, indefinite, fine) and 我把那本书看完了 (把 needs the SPECIFIC 那本书 AND a result 完) are different constructions; 我把一本书看了 is ungrammatical.

Learners assume 被 can be added or removed without changing who did what. WRONG: 被 changes WHICH noun is the subject, and so whose perspective the sentence tells. ✓ 猫被喂饱了 (Māo bèi wèi bǎo le, the cat was fed until full) states the passive, whereas 猫喂饱了 relies on the reader inferring it from context. When a task asks for the affected party's perspective, use 被.

把 can only front an object both speaker and listener already know -- a SPECIFIC thing, never a vague "a/an" one. ✗ 我把一本书看完了。("a book" is indefinite.) ✓ 我看完了一本书。(Wǒ kàn wán le yì běn shū.) — I finished reading a book. But once that SAME book is known: ✓ 我把那本书看完了。(Wǒ bǎ nà běn shū kàn wán le.) — I finished reading that book (那本书 is now specific).

A 把 sentence is not finished by the verb alone -- the point of 把 is to say what RESULT happens to the object, so the verb needs a complement, a quantity, or at least 了. ✗ 我把饭吃。(what happened to the meal is unstated.) ✓ 我把饭吃完了。(Wǒ bǎ fàn chī wán le.) — I finished eating the meal. The same after 被: ✗ 自行车被偷。 ✓ 自行车被偷走了。 — The bicycle was stolen.

Some tasks require the answer from the perspective of the thing the action happens TO, not the doer -- a plain active sentence can lose a perspective mark. ✗ 主人每天喂它两次。(told from the OWNER's view when the PET's was required.) ✓ 它每天被主人喂两次。(Tā měitiān bèi zhǔrén wèi liǎng cì.) — It is fed twice a day by its owner (被 makes the pet the subject).

In a 被 sentence the agent (the doer) comes right after 被 and BEFORE the verb, never after it. ✗ 那本书借走了被他。 ✓ 那本书被他借走了。(Nà běn shū bèi tā jiè zǒu le.) — That book was borrowed by him. The order is fixed: affected-subject + 被 + agent + verb + result. Putting 被 + agent at the end scrambles the sentence.

When a 把 sentence places an object somewhere, use the verb + result-location complement 放在…上, and keep object and location in order. ✗ 把书桌子上放了。(object and location muddled, no clear result.) ✓ 我把书放在桌子上了。(Wǒ bǎ shū fàng zài zhuōzi shàng le.) — I put the book on the desk. 放在…上 tells you exactly where the specific object 书 ended up.

A directional result complement must be complete: use 回来 / 回去 / 出来, not a bare 回 / 出. ✗ 想把书找回。 ✓ 想把书找回来。(xiǎng bǎ shū zhǎo huílái) — want to get the book back. The full 回来 states the direction and result; the bare 回 leaves the 把 clause incomplete, exactly the trap 把 sentences are meant to avoid.