How to remember which word should add "儿" at the end ?

Take notes whenever you hear the 儿 if you have good listening. And try to use them when you speak Chinese.

Don‘t worry about “儿“ at the end . Most of time it is used to soft the tone. If you don't know whether you need add 儿 or not, then don't add. People in Peijing use this word more often, and lots of cities don't use it.

那人 , 这儿

There‘s no rule，I'm afraid.

You can use this document to look up.

it's hard to remember, as a Chinese, we never remember this kind of rule. read and listen a lot authentic material in Chinese.