YOU SAID:
I dont know how to use this app...
INTO JAPANESE
私はこのアプリを使用する方法を知らない.
BACK INTO ENGLISH
I don't know how to use this app.
INTO JAPANESE
私はこのアプリを使用する方法がわからない。
BACK INTO ENGLISH
I don't know how to use this app.
That didn't even make that much sense in English.