Let’s dive into how to use the particle 了 (le) as an aspect particle to express completion and as a modal particle to show a change of state.
Supplementary vocabulary
(1) 了 as an aspect particle to express completion
Firstly, we can use 了 (le) to signify that an action is finished. It indicates an action’s completion within the discussed timeframe, which can be in the past, present, or future. Keep in mind, it’s about action completeness, not when it occurred. Even though we use it most often with actions that have taken place in the past, it’s not the same as the past tense. Aspect 了 is placed directly after the verb, so it is also referred to as verb 了.
Subject + Verb + 了 + Object
1. 我打扫了房间。(wǒ dǎsǎo le fángjiān.)
I cleaned the room.
2. 他唱了一首歌。(tā chàng le yīshǒu gē.)
He sang a song.
3. 她买了一个新外套。(tā mǎi le yīgè xīn wàitào.)
She bought a new jacket.
Though these examples are correct as is, it’s important to note that when talking about completed actions we will often use something called a result complement after the verb. E.g. 我吃完了。(I finished eating). You will learn more about this in an upcoming lesson.
（一）Specifying when something was completed
Subject + Time + Verb + 了 + Object
Time + Subject + Verb + 了 + Object
4. 我早上喝了一瓶可乐。(wǒ zǎoshang hē le yì píng kělè.)
I drank a bottle of Coke this morning.
5. 她昨天对我微笑了。(tā zuótiān duì wǒ wēixiào le.)
She smiled at me yesterday.
6. 这个月我读了五本书。(zhègèyuè wǒ dú le wǔ běnshū.)
This month I read five books.
（二）Expressing how long something was done for
We still place 了 (le) after the verb and then indicate the duration followed by the object (if any).
Subject + Verb + 了 + Duration + Object
7. 她看了一晚上电视。(tā kàn le yì wǎnshang diànshì.)
She watched television all night.
8. 他们学习了一天。(tāmen xuéxí le yìtiān.)
They studied for the entire day.
9. 我学了两年汉语。(wǒ xué le liǎng nián hànyǔ.)
I studied Chinese for two years.
(2) 了 as a modal particle to show a change of state
Secondly, we can use 了 (le) to indicate a change in the situation, highlighting that things are different now compared to before. When used in this way, we place了at the end of the sentence.
Subject + Adjective + 了
1. 我胖了。(wǒ pàng le.)
I got fatter. (I gained some weight.)
2. 我饿了。(wǒ è le.)
I’m hungry.
Subject + Verb [phrase] + 了
3. 我开灯了。(wǒ kāidēng le.)
I turned on the light.
4. 下雨了。(xiàyǔ le.)
It’s raining now.
Subject + 不（没 ）+ Verb + 了
5. 她不去了。(tā bú qù le.)
She isn’t going anymore.
6. 我不学了。(wǒ bù xué le.)
I’m not studying anymore.
7. 我的手机没电了。(wǒde shǒujī méidiàn le.)
My phone ran out of charge.
(3) Using two types of 了
Some sentences may contain 了(le) twice when there is both a verb 了 and a sentence 了. This occurs when we want to express both a completed action and change of state. It shows what has been completed “so far” or “up till now”.
1. 他已经喝了六瓶啤酒了！(tā yǐjīng hē le liù píng píjiǔ le!)
He has already drunk six bottles of beer!
2. 我都学习了十个小时了。(wǒ dōu xuéxí le shí gè xiǎoshí le.)
I have been studying for ten hours already.
3. 我的狗已经睡了一个上午了。(wǒde gǒu yǐjīng shuì le yīgè shàngwǔ le.)
My dog has been sleeping all morning.