if in a question constraints are given.

There are two subtasks.

subtask 1 - say some contraint

subtask 2 - original constraint

here original constraint is applicable in subtask 1 or not??

No, the test cases contained in Subtask 1 will be according to the constraints given for it

Example, if given N<=1000 for subtask 1

Then all the test cases will have N less than 1000 for that subtask