了 is simple in concept, but hard (for me at least) to use perfectly. There are a few main grammatical usages: To show that a situation has changed: 我老了 I'm old (now). This implies I wasn't old before 下雨了 It's raining (now). This implies it wasn't raining before 孩子累了 The child is tired (now). This implies he/she wasn't tired before To show that something is about to happen, used together with 快（要） 我們快到了 We'll be there soon 我的女朋友快要過生日了 It'll be my girlfriend's birthday soon To express "already", usually used with 已經 我（已經）有女朋友了 I already have a girlfriend 他（已經）走了 He's already left To express not anymore 你不喜歡我了嘛？Don't you like me anymore? 手機沒電了 My phone is out of battery To show a completed action 今天我吃了早飯 I ate breakfast today 她上個禮拜去了北京 She went to Beijing last week To express that something has been ongoing and will probably continue, like "have been ___ing" 我在香港住了兩年了 I have been living in Hong Kong for 2 years 你打電話打了兩個多小時了 You've been on the phone for over two hours already Used with 完 to show something is finished 我吃完了 I'm finished eating 我看完了 I've finished watching it Used with 差一點 to express almost 我差一點就輸了 I almost lost 他差一點就死了 He almost died As a result compliment 我寫錯了 I've written it wrong 杯子摔壞了 The glass is broken To express never again with 再也 我再也不想見到你了 I never want to see you again 大家再也不會相信老師了 Nobody will ever believe the teacher again In structures like 別___了；太___了 別喝了，你已經醉了 Stop drinking, you're already hammered 我太累了 I'm exhausted Complements like 死了；極了 今天熱死了 It's so unbelievably hot today 這道菜好吃極了 This food is so good I haven't included everything, but that's enough to get started with Laurence

Laurence explained very well and was comprehensive.👍

There are two meanings with 了. First, it can be expressed as "the result of behavior (action)", or "things have been completed." In this case, their tone is generally positive and usually appears at the end of the word. For example: 坏了; 完了; 好了; 行了. In addition, 了 can be used as part of a word. For example, 了解, 了断, 了结.... these words generally have the meaning of termination except '了解'