Because the 'shì 是 ... de 的 ' construction is associated with completed actions and because English uses the past tense to report things that have taken place, students often make errors by focusing on completion when circumstantial elements about an action actually demand the attention. It is important to remember that the 'shì 是 ... de 的 ' construction does not deal with the completion, duration or frequency of an action or with the quantity of the object. It focuses on when, where and how an action took place. It is used to seek or provide information about the time, place, company, conveyance, manner, tools, purpose, agent and recipient of the action. The following dialogue shows when to use le 了 and when to use the 'shì 是 ... de 的 ' construction.

1. 
   Nǐ mǎi shū le ma? (Focus: accomplished fact)
   你买书了吗？
   Have you bought the books yet?


2. 
   Mǎi le. (Focus: accomplished fact)
   买了。
   Yes, I have.

1. 
   (Shì) shénme shíhou mǎi de (Focus: time)
   (是) 什么时候买的？
   When did you buy them?


2. 
   (Shì) zuótiān mǎi de. (Focus: time)
   (是) 昨天买的。
   I bought them yesterday.

1. 
   (Shì) zài nǎr mǎi de (Focus: place)
   (是) 在哪儿买的？
   Where did you buy them?


2. 
   (Shì) zài Xīnhuá shūdiàn mǎi de (Focus: place)
   (是) 在新华书店买的。
   I bought them at the Xinhua Bookstore.

1. 
   Xīnhuá shūdiàn hěn yuǎn, nǐ shì zěnme qù de (Focus: conveyance)
   新华书店很远，你是怎么去的？
   Xinhua Bookstore is far, how did you go there?


2. 
   (Shì) zuò chē qù de (Focus: conveyance)
   (是) 坐车去的。
   I went by bus.

1. 
   Zuò-le duō cháng shíjiān? (Focus: the duration of the action)
   坐了多长时间？
   How long was the ride?


2. 
   Zuò-le yí ge zhōngtóu de chē. (Focus: the duration of the action)
   坐了一个钟头的车.
   The ride was an hour long.

1. 
   Nǐ (Shì) yí gè rén qù de ma? (Focus: company)
   你(是) 一个人去的吗？
   Did you go there alone?


2. 
   Bú shì (yí gè rén qù de). (Focus: company)
   不是(一个人去的)。
   No, I didn't go there alone.

1. 
   (Shì) gēn shéi qù de (Focus: company)
   (是)跟谁去的？
   With whom did you go there?


2. 
   (Shì) gēn Wáng Tóng qù de. (Focus: company)
   (是) 跟王同去的。
   I went with Tong Wang.

1. 
   Tā yě mǎi shū le ma? (Focus: accomplished fact)
   他也买书了吗？
   Did he buy books, too?


2. 
   Tā méi mǎi. (Focus: accomplished fact)
   他没买。
   No, he didn't.

1. 
   Nǐ mǎi-le jǐ běn (Focus: quantity of the object)
   你买了几本？
   How many did you buy?


2. 
   Wǒ mǎi-le wǔ běn (Focus: quantity of the object)
   我买了五本。
   I bought five.