The le particle is used to talk about completed actions. It is used with those functive verbs (action verbs) which express actions which can be completed. The le particle cannot be used with certain functive verbs such as rènshi 認識 (be aquainted with); ài 愛 (to love); or xiǎng 想 (to think) because these actions can never be considered finished.
How do we express these unending functive verbs in the past tense?
Stative & Equative Verbs
Mandarin Chinese has a class of verb called stative verbs, which are used to describe a state which is relatively unchanging. Examples of stative verbs include gāo (tall), gùi (expensive), and dà (large).
Equative verbs express the idea that two things are the same – “He is my teacher.”
The le particle cannot be used with either stative or equative verbs, so how do we express these ideas in the past?
When we want to use a stative verb or an unending functive verb to talk about the past, we can use a time expression like shàngge yuè (last month), xiǎode shíhòu (when a child), or niánqīngde shíhòu (when young). Time expressions usually occur at the beginning of a sentence, either before or after the subject.
These time expressions may not be needed if the context of the conversation is clear. A sentence like “wǒ shì xué sheng” could mean “I am a student” or “I was a student,” depending on the context.
Examples of Time Expressions
Tā shì wǒ gāozhōng shíhou de lǎoshī.
He was my high school teacher.
Wǒ shànggè yuè hěn máng.
I was very busy last month.