Although Mandarin does not have verb conjugations as in many other languages, there are ways to express completed actions as well as events which occur in the past or future.
The easiest way to do this is to indicate a time frame for the action being discussed, as in the sentence “Tā zuótiān qù Táiběi.” 他昨天去台北｡ This would simply mean that he went to Taipei yesterday.
However, if he had an obligation to go to Taipei, and he fulfilled that obligation yesterday, you could use “le” - “Tā zuótiān qù Táiběi le.” 他昨天去台北了｡
The “le” Particle
If you wish to express the idea that an action occurred in the past and is finished, you can use the “le” 了particle in combination with a functive verb (an action verb).
“Le” is used after the functive verb, either directly after the verb or at the end of the sentence. It is not needed if the context of the conversation makes the meaning clear.
Tā chī bǎo le.
He has already eaten.
Chén xiǎojie jīntiān mǎi dōngxī le.
Miss Chen already bought some things today.
Wǒ zǎoshang mǎi shū le.
I bought the book this morning.