Dear student,

In jee mains the maximum number of attempts allowed are three. You first attempt is counted as over in the year that you have passed your class 12th. After that you can only give two attempts only in next consecutive years.

Also it doesn't matter that whether you have given the jee mains or not your attempt will be counted as over.

Now if you are in the ending of first year or starting of second year, and after class 12th ( if you have passed your class 12th in 2019 ), then the next year will be your 3rd consecutive year, then that means, still you have one attempt left, and you can give jee mains 2021. And if you got rank and get a good nit, then definitely you can join it.

But the admission will be given from a new session, which means you will again study from first year.
And only after completion of first year you can get second year.

So i hope you doubts are cleared.