can i appear for jee mains 2018 even if i have not scored 75% in the 12th board isc???

You are not eligible.

According to MHRD, apart from admissions to IITs, the qualifying marks for admission to NITs, IIITs and GFTIs have also been revised.

As per the notification, for candidates to qualify for admissions to NITs/IIITs/GFTIs, they should have secured at least 75% marks in class 12th exam or be in the top 20 percentile in the class 12th exam conducted by the respective boards.

For SC/ST candidates, the eligibility criteria is 65% marks in class 12th.

Rest educational, age criteria and other eligibility you can check here:

JEE MAIN eligibility

All the best!!

Yes, you can appear in JEE Main if you have not secure 75% marks in your boards exam.

In order to get admission in NITs/IITs/IIITs anf GFTIs you need to get 75% marks in your board exam or you should be in top 20 percentile.

Below is the eligibility criteria you can check

1- Aspirants should have passed 12th class or qualifying exam in 2015 or 2016.

2- Students, who are appearing in class 12th or qualifying exam in 2017 can also apply.

3- It is mandatory for aspiring candidates to have taken 5 subjects in 12th or qualifying exam.

4- The 5 subjects for B.Tech/B.E. are:
1-Language, 2-Physics, 3-Mathematics, 4-Any one of (Chemistry, Biology, Biotechnology, Technical Vocational Subject), 5-Any other subject.

5- According to MHRD, apart from admissions to IITs, a. As per the notification, for candidates to qualify for admissions to NITs/IIITs/GFTIs, they should have secured at least 75% marks in class 12th exam or be in the top 20 percentile in the class 12th exam conducted by the respective boards. For SC/ST candidates, the eligibility criteria is 65%
.
To check the complete eligibility criteria you can refer below link

JEE Main Eligibility Criteria

Good Luck!!