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