HOW TO USE 
The particle le is mainly used to specify when and how the action expressed by the verb takes place, and it is one of the most useful. Since it is not always used according to fixed rules, Chinese themselves are not always able to explain why a sentence requires it or rejects it. Below, some of the most common contexts one can find it in.
1. Past tense: |
|  | I bought three books. |
|  | (the past tense is stressed) |
| N.B.: here le indicates that the action is already finished. If the object is preceded by a numeral, then put le between verb and object, and never after the object, as shown below: |
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| If the object is very short, you can express the past tense either putting le between verb and object or after the object: |
|  | (the past tense is stressed) |
|  | (the tone is more neutral; in this case, le indicating the past tense would coincide with a le indicating a change) |
| If the action happens on a regular basis, is repeated more than once or it is frequent, never use le: |
|  | I often go eat there. |
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| 2. Present: |
|  | What time is it? |
|  | What day of the week is it today? |
| N.B.: these sentences are quite difficult to explain; just use them! |
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| 3. Future: |
|  | It will rain soon. |
|  | I have almost finished writing. |
|  | I was just leaving. |
| N.B.: always with jiu, yao, kuai (it indicates that something is going to happen or that the action will end in a few time). |
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| 4. To indicate a change: |
|  | Look, it started to rain. |
|  | (Now) I understand! |
|  | The weather got colder. |
| N.B.: this kind of le indicates a new fact and it is often used with adjectives to indicate a quality that the subject didn't have before. |
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| 5. To indicate that the action continues: |
|  | I've studied Chinese for three years (and I'm still studying it). |
| N.B. used with a specification of time (see card 6). |
