What should i do not to forget the vocabularies? Any advice?
You can use flashcards, there's an online flashcard site quizlet.com!.. You can also read english books and watch english movies. Also, just overall conversing with other people helps!
From my experiences, ways of learning and remmebering vocabulary is understanding the words in contexts ( learning through examples) and keep practicing. One great point is learning group of words that usually go each other, it helps you sounds more advanced and more natural.
Ex: some say "yellow hair", actually it is "blond hair", as "yellow hair" is not wrong, just not as precise as "blond hair".
Make sentences... and use them.
Relate the new word to a sentence, a situation or an object. It will be easyer, I guess.
But are you talking about difficult vocabularies in your language or every word in another language?
I suggest you to use Anki flashcard to review all the words you know :) It does works.