Hi! If I were you I would look for any similarities or associations with you mother tongue vocabulary. Or it depends what kind or memory you have? If it's visual one then make some cards with pictures. Or if it's easier for you to remember things you hear record the words and listen to them while you're on a bus or in a car. And other good thing is to write the word like 20 times or even more. Then your hand will know the movement of the pen on the paper. Sometimes to know how to spell a word I need to write it. Anyway, hopefully it was useful.

I suggest you to read a book of your favorite movie or subject and watch a movie with English subtitle. My experience is I usually watch my favorite series with an subtitle, and pause the movie to translate from dictionary when unknown word comes up while I am watching them. And when the word comes up repeatedly my brain memorize them easily. Also I like Harry Potter movie from my childhood and waiting for the next part of the move for a year seemed to me so intolerable so used to read its Book before the next part of the movie comes out. Reading the book was not easy for me first but insistent reading and translating gave an opportunity to memorize the words.

If you want to remember them, just practice using them on a daily basis. That would probably be the best way to remember. Also practice writing them often also. Keep them on index cards if that would help!

You simply just cannot remember. It's normal to not remember exactly what something means even as a native speaker. I would recommend making flashcards of the words you forget or use the website called Quizlet. You can make flashcards of the English words and have the definition on the back. Once you make them, quiz yourself. This can help you with remembering definitions but with spelling I would recommend writing lots of sentences using the word to get use to using it. Good Luck!