The most challenging part of learning English is using appropriate words in English. Although I have started learning English since I was 9, it`s too difficult to choose words and expressions to say in English. There`s lots of idioms that fit more in the specific situation, but I`m short of idioms to use. When I watch TV shows or movies in English, sometimes it`s very hard to understand without the subtitle in Korean because they use the idioms that I can not guess from the meaning of the word in dictionary-because it`s kind of expression that has history-like 'in the shoe(it`s the example of idiom'. Especially when I watch the sitcom-like the "FRIENDS" or "Brooklyn 99", the characters use the expressions or slangs that they casually use in their life, but not in the textbooks. I want to develop this sense to understand more and use the appropriate expressions that suits the situation.

Thank you for sharing this challenge you have experienced in learning English, Linda. We'll do our best to develop this skill. -Faith-

The most challenging part of learning English is using appropriate words in English. 

>> CORRECT

OR The most challenging part of learning English is using the appropriate words.

Although I have started learning English since I was 9, it`s too difficult to choose words and expressions to say in English. 

>> Although I started learning English when I was 9, it’s still difficult to choose the right words and expressions.

There`s lots of idioms that fit more in the specific situation, but I`m short of idioms to use. 

>> There are many idioms that fit specific situations, but I’m short on idioms to use.

When I watch TV shows or movies in English, sometimes it`s very hard to understand without the subtitle in Korean because they use the idioms that I can not guess from the meaning of the word in dictionary-

>> When I watch TV shows or movies in English, it’s sometimes very hard to understand without Korean subtitles because they use idioms that I cannot interpret from the dictionary definitions.

because it`s kind of expression that has history-like 'in the shoe(it`s the example of idiom'. 

>> In fact, it’s the kind of expression with historical roots, like the idiom 'in someone else's shoes.'

Especially when I watch the sitcom-like the "FRIENDS" or "Brooklyn 99", the characters use the expressions or slangs that they casually use in their life, but not in the textbooks.

>> Especially when I watch sitcoms like 'Friends' or 'Brooklyn Nine-Nine,' the characters use expressions or slang that they casually use in their daily lives, but not in textbooks.
I want to develop this sense to understand more and use the appropriate expressions that suits the situation.

>>I want to develop this sense to better understand and use appropriate expressions that suit the situation.