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When do you use 了／儿？

了 is being uesd when you want to say something in past tense; and we usually put it at the end of sentence. For examples:
他走了- He left
他来了- He came or He has come
我做完我的作业了- I finished my homework.

儿 is being used in some informal conversation or by the people from the north part of China; and we also put it at the end of sentences, mostly, which are interrogative. For example:
你在哪儿? - Where are you?
However, we do not use 儿 in formal conversation, even though it is the conversation between two northern people.

儿means that a action u have already done ，just such as：他走了，he left。

了：助词
1.用在动词或形容词后， 表示动作或变化已经完成
read twice;
读了两遍
Apples got red.
苹果红了。
2.用在句子的末尾或句中停顿处， 表示变化， 表示出现新的情况， 表示催促或劝止
The spring comes and peach blossoms [flowers] are all open.
春天了， 桃花都开了。
I don't go out whenever it rains.
天一下雨， 我就不出门了。