The 把 (bǎ) sentence is a useful structure for focusing on the result or influence of an action. It's really common in Mandarin, but can feel a bit strange for English speakers at first.
A 把 sentence (把字句 in Chinese) is a sentence in which the object of the sentence (the noun that is having some action done to it) appears after the subject and before the verb, preceded with a 把.
Remember, a basic sentence in Mandarin is formed with a subject-verb-object (SVO) word order, as in English:
subject+verb phrase +object
A 把 sentence shakes things up a bit, and you get this structure:
subject+把+object+verb phrase
Note that the object has moved, and is preceded by 把. You now have SOV word order.
This is all well and good, but most students of Chinese, on learning about 把 sentences for the first time, have the same reaction: "Why the hell would I ever use this structure? The SVO word order always works just fine, right?" The answer is: well, no... not always. It's true that 把 sentences are often used to achieve somewhat subtle differences in emphasis, but there are also very good reasons to use 把 sentences when a regular sentence just won't do. Take this sentence for example:
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你 把 书 放 在 桌子 上 。
How would you say this without the 把 construction? You might try this:
× 你 放 书 在 桌子 上 。
The only problem is that the above sentence is not grammatical. You can't put an object right after a verb, and then put other modifiers of the verb after the object. Below, we'll go into more detail about when and how to use 把 sentences.