How to study for the exams which are mathematical in approach but subjective in nature? Predominantly whenever we are to face objective questions we generally tend to practice more and more objective question papers and sort of "remember the mistakes that we make while solving them".  However when it comes to subjective papers this approach doesn't seem to work for me anymore.  The more subjective questions I study the more I tend to deviate from whatever I need to retain for the approach.  Any suggestion about this will be highly appreciated.  I am particularly speaking with regard to semester examination of an Undergrad/Postgraduate course in Material Science &amp; Engineering.
Learning math, science and engineering is to a certain extent about learning a collection of computational techniques that one can improve at by rote practice - integrating, differentiating, solving equations etc. But to a much larger extent it is about acquiring a conceptual understanding of the key ideas and concepts of the science you are being taught. This is a higher lever skill, and one that is ultimately much more important.

What you call “subjective” questions are precisely those questions that are designed to test your understanding. They are the questions that, ever since I started teaching, I have been hearing students (even well-intentioned, hard-working, eager to learn students) describe as “trick questions”, “unfair”, “not in the book”, “impossible to study for”, and so on. They are the questions that students dread and loathe and complain about and seek to find ways to avoid and bluff their way through. They are the questions that really test whether you have learned the main thing we wanted you to learn.

To be fair, subjective questions are indeed hard, and I completely sympathize with your frustration about them. They are characterized by the fact that there is no mechanical practice procedure, like “solve X problems from section Y in the book”, that is guaranteed to prepare you for them. Learning how to study for them is probably the single most valuable skill a student can hope to learn in their time at university. If you have learned how to study for subjective questions, it means you have learned how to learn, and that’s a wonderful thing to know.

So how do you study for them? In some sense that’s something each person has to discover for themselves. But in another sense the answer is blindingly obvious: you study the material, and you think about it. That’s it! But what I mean is really really study the material, and really think about it: that is, you think critically about each and every sentence of each and every section in the book/lecture notes and each and every thing the instructor said, trying to identify what conceptual meaning it is trying to tell you. You try to probe into every nook and cranny of the theory by criticizing it, and asking “what if” questions and “does this make sense?”, and reading Wikipedia and textbooks and watching online videos. You solve problems, and solve more problems, then go back to the theoretical part of the textbook and read it again and think about it in the context of the problems you solved. And watch more videos and read more books and forums and talk to your friends about the material. Etc etc. - note that this process has no natural end because no one understands everything, not even your professors. But at some point you’ll probably feel like you understand things at a reasonably good level for the exam you are about to take.

And then you move on to the next subject and start all over again. :-)