Is there a way for foreigners in Gangnam to easily memorize small words?

Hello! It's been a little over a year since I came to Korea. I am currently studying Korean in Gangnam, but I find it really difficult to memorize small words during class time. For example, in a sentence like 'I learned a lot of new expressions while watching Korean dramas,' I have trouble remembering words like 'expressions.' Are there others who struggle with memorizing small words like me? I have tried making vocabulary lists and using flashcard apps, but I easily forget after 2-3 weeks, which is frustrating. If anyone knows of good study methods or study places near Gangnam, please recommend them. Your advice would be greatly appreciated! Thank you!