In the following situations the particle "了 le " for completed actions is not used.
1."了 le " is not used for prolonged and regular actions in the past. In other words, if the sentence contains words such as 每天everyday ,年年every year,常常 always (měi tiān nián nián chánɡ chánɡ )... the particle "了 le " for the completed action is not used.
wǒ yǐ qián zài zhōnɡ ɡuó jiāo shū
我 以 前 在 中 國 教 書。
I IN THE PAST IN CHINA TEACH
I taught in China in the past.
2. "了 le " is not used in sentences which have modal verbs such as 應該should (yīnɡ ɡāi ), 得have to (dei), 可以can (ké yǐ ), 能can (nénɡ ), 會will (huì,) 要be going to (yào ), 想want (xiǎnɡ) ,
or for verbs which indicate feelings such as 愛love(ài) ,喜歡like(xǐ huɑn), 覺得(jué de) think, 知道(zhī dào) know, 願意(yuàn yì) be willing to , 肯(kěn)be willing to .
zuó tiān nǐ yīnɡ ɡāi qù kàn tā
昨 天 你 應 該 去 看 他.
YESTERDAY YOU SHOULD GO SEE HIM
You should've gone to see him yesterday.
shí nián qián tā hěn xǐ huɑn chī fǎ ɡuó cài
十年 以前 她 很 喜 歡 吃 法國 菜.
TEN YEAR AGO SHE VERY LIKE EAT FRENCH FOOD
Ten years ago she liked eating French food very much.
3. 「了le」 is not used when 是shì , 在zài , 有yǒu* are the main verbs of sentences.
shànɡ xīnɡ qī tā zài běi jīnɡ
上 星 期 他 在 北 京。
LAST WEEK HE WAS (IN) BEIJING
He was in Beijing last week.
If 有 yǒu(to posses) is used as the main verb of a sentence, "了 le "(for completed action) is not used. However, 了 can be used if the object of a sentence is an abstract one which has an implication of action, such as 「development」, 「improvement」 他的學習有了很大的進步. He has improved greatly in his studies.
4. "了 le " is not used for direct and indirect speeches.
dir. speech
zuó tiān tā wèn wǒ nǐ yào qù zhōnɡ ɡuó mɑ 昨天 他問我: "你 要 去 中 國 嗎?"
YESTERDAY HE ASK ME: "YOU WANT GO CHINA MA?"
Yesterday he asked me: "Would you like to go to China?"
indir. speech
zuó tiān tā wèn wǒ yào qù zhōnɡ ɡuó mɑ
昨 天他 問 我 要 去 中 國 嗎.
YESTERDAY HE ASK ME WANT GO CHINA MA
Yesterday he asked me if I would like to go to China.
As you can see, in the situations when the completed action 了 is not used, time-words are used to indicate the actions happened in the past.