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Meanings and usage of words can be remembered well only when you use them often.  Otherwise, you will forget the vocabulary you have learnt after a few days.

You have to write  essays or rewrite your exercises once or twice using the words you have learnt.   Grammar is not easy.  You have to practice speaking after you have sufficient practice in writing grammatically correct English.

With out writing three to four times and using them verbally, it is not possible to remember the usage.   Do not be embarrassed if you make mistakes.  practice speaking.

do hardwork
regularly revise the words before that buy a dictionary and revise and see the pronounciation and the meaning of difficult found in your own text or news paper. don't hesitate or don't be lazy to see the meanings that you do not know. refer internet for absolute pronounciations 
and if you have grammatical errors please refer your teachers. the teachers show the best way to us.they are our life guiders. revise the difficult words once a day in the morning or in the evening atleast learn  1 meaning a day .
hope it helps you. 
                                                             thank you