Which are the main subjects in Bachelor of Technology?

All the subjects are important but if you talk about the tough subjects are taught in the 3rd year.

The main subjects for first year are same for every one. The main core subjects depends on what field have you chosen. and moreover Bachelor of Technology is B.Tech which is further classified into other main domains. If you wish to go for Oil and Gas informatics or Cloud Computing engineering the

There are no main subjects. All the subjects are equally important and you will have to study all of them.