when will form of air force y class come

Hello Kartik!
If your query is about selection process in the Indian Air Force to join as Airmen in Group-Y, regulated by the Central Airmen Selection Board, then the online application form for intake - 01/2021 was available from 2nd January, 2020 to 20th January, 2020.
The online examination - STAR 01/2020 was scheduled to be held from 19th March to 23rd March, 2020 for the various groups. But it has been postponed due to COVID-19 outbreak. New dates will be announced by the last week of May.

So, the application date is over for this year.

For the next session, kindly make sure to get the updates on time by regularly visiting :
http://airmenselection.cdac.in/CASB/
Or http://www.careerindianairforce.cdac.in

The advertisement for recruiting the candidates for IAF is usually published in the second week of December every year.
So, keep checking the official links provided above.
Basically, you need to first go carefully through the advertisement released on the official website, check your eligibility and other medical requirements and then fill up its online application form provided at : http://airmenselection.cdac.in before the last date.
Stay connected to us for not missing such important updates from the next time.

Hope this answer will help you in some way.