last sunday i had my test of physics.All day and night i keept brushing up my concepts.i was all prepared but when i sat down for the examination i got lost in the paper.my objective was all good(i scored 100% ) but in subjective exam i was blank.It happens with me everytime esp when the paper constst only of derivations.No matter how well i do them i forget them during the exam and loose my position.

Is there any way to tackel this?

Perhaps an example of what you mean would help? If you are talking about reciting stuff (derivations, definitions, ...) that were given in the course/lecture/class/...then a standard way of preparing is making cards with the respective keywords, mix the cards, draw them at random, write down the derivation/definition/... for the keyword and, in case you cannot, look it up. This procedure is then repeated until you can write down them all without looking something up (potentially by removing cards which become too boring in between). Not sure if that is what you are talking about, so ... see my first sentence (note also that terms like "objective exams" and "subjective exams" might have little to no meaning for people not in the same country/area/school/education level as you).

objective exams reffer to multiple choice questions and subjective reffers to therotical questions.

i'm a bit comfortable with all sort of derivations at home but problem arises during exam times.