Past tense in Mandarin When you write a sentence in chinese where you indicate with an adverb the time of the action for example： 昨天我吃了炒饭 上星期我去了北京 is it necessary to add the particle 了 to indicate that the action was completed or can you omit it and just write 昨天我吃炒饭 上星期去北京 note: I want to know which one of the past two examples are correct the first or the second and why