As Chinese has no past-tense form for verbs, 了 can act, of sorts, as a substitute. However, instead of explicitly saying, “this verb is past tense,” it says, “this is a completed action.” This might seem like a trivial distinction, but it’s important when entering the nuanced world of 了. Let’s take a look at some of those, from most common (and easiest!) to some more obscure, baffling ones.
verb + 了
When used to state that an action has been completed, the 了 generally comes directly after the action.
Since 了 indicates a completed action, it can also refer to actions that will be completed in the future - not just the past! This is useful when talking about a sequence of things that will happen.
verb + 了
Just as an action being completed is one particular change of state (uncompleted to completed), 了 can also indicate and emphasize other changes of state. In these cases, 了 generally comes at the end of the sentence or clause but will sometimes come right after the verb, too.
I used to be a shorty.
She used to be a vegetarian.
This indicates that I didn’t like you before (sorry), but, hey, I do now!
快 + verb/adjective + 了
When paired with 快, 了 can indicate that an action or change is about to happen.
太 + adjective + 了 可 + adjective + 了 adjective + 多了
了 can pair with certain intensifiers to indicate excess.
Command + 了
Incredibly, 了 can also be used to tone down a command, much like the particle 吧. If we want to sound less forceful when making a command, we can add 了 to the end.
verb + 了 + Object + 了
If you see 了 twice in one clause, it’s generally either for emphasis, expressing duration (see next section), or both.
verb + 了 + duration verb + 了 + duration + 的 direct object
As mentioned, 了 indicates a completed action when placed directly after a verb. If you want to say how long that action had been going, add a period of time after 了.
Note that in the above examples, the actions have definitely ended. You can add a second 了 after the period of time to specify that the action is still ongoing.
verb + 了 + duration +了 verb + 了 + duration + 的 direct object + 了
Note that duration, whether with one了 or two, can be measured in things other than time, take the example from above:
We’re measuring duration with number of glasses drunk. She’s already had ten glasses. She’s plastered. And the final 了 shows that she has no intention of stopping unless acted on by an outside force.
verb + 得了 verb + 不了
In some cases, 了 even has a different pronunciation: liǎo. This is mostly when placed after 得 or 不 to show whether an action is able to be completed 得了 or unable to be completed 不了.