Introduction:
JEE Main is one of the most popular engineering entrance exams in India. It is conducted by the National Testing Agency (NTA) for admission to undergraduate engineering courses in various colleges and universities in India.

Improvement Exam:
An improvement exam is an exam that is taken by students who wish to improve their scores in one or more subjects in class 12th. It is conducted by the respective State Board of Education.

Eligibility for JEE Main:
To be eligible for JEE Main, a student must have passed class 12th or its equivalent examination with Physics, Chemistry, and Mathematics as mandatory subjects. The minimum aggregate marks required for eligibility is 75% for General category students and 65% for SC, ST, and OBC students.

Impact of Improvement Exam on JEE Main Eligibility:
If a student wishes to improve their score in one subject only and has already passed class 12th with the minimum required aggregate marks, then they do not need to appear for the improvement exam in all the subjects. They can appear for the improvement exam in the subject they wish to improve and use the marks obtained in that exam for JEE Main eligibility.

However, if a student has not passed class 12th with the minimum required aggregate marks, then they must appear for the improvement exam in all the subjects to improve their scores and become eligible for JEE Main.

Conclusion:
In conclusion, it is not necessary for a student to give the improvement exam for all subjects in class 12th to be eligible for JEE Main 2018 if they wish to improve only one subject. They can appear for the improvement exam in the subject they wish to improve and use the marks obtained in that exam for JEE Main eligibility.

No one sub applicable,,,