L. A verb that is in the 'shì 是... ...de 的 ' construction does not take the verb-suffix le 了. The 'shì 是... ...de 的 'construction is used to emphasize when, where and in what manner an action took place. This construction does not focus on the completion of the action. Instead, it focuses on the circumstances around a completed action. Therefore, le 了 is not used in the 'shì 是... ...de 的 'construction.
Note: (x) means that -le 了 may not be used there.


    1. Nǐ shì shénme shíhou lái (x) de?
      你是什么时候来的？
      When did you come?


    2. Wǒ shì jiǔ yuè lái (x) de.
      我是九月来的。
      I came in September.

      Person A already knows Person has come back, so the completion of the action 'come ' is not the focus. Person A is only interested in the time when Person came back. This is why le is not used.


    1. Nǐ shì zěnme lái (x) de?
      你是怎么来的？
      How did you come?


    2. Wǒ shì zuò huǒchē lái (x) de.
      我是坐火车来的。
      I came by train. /I took the train.

      Person A already knows Person B has come back, so the completion of the action 'come ' is not the focus. Person A is only interested in the means of transportation by which Person B came back. This is why le 了 is not used.


    1. Nǐmén shì zài nǎr chī (x) de?
      你们是在哪儿吃的？
      Where did you eat?


    2. Wǒmén shì zài Guǎngdōng Fànguǎnr chī (x) de.
      我们是在广东饭馆儿吃的。
      We ate at the Canton Restaurant.

      Person A already knows Person B has eaten, so the completion of the action 'eat ' is not the focus. Person A is only interested in the place where Person B ate. This is why le 了 is not used.