Use 'impracticable' to describe tasks that can't be done due to a lack of materials, money, or other necessary items. Building a new school was impracticable with the budget cuts.
Pro Tip 2/3
Physical Constraints
Use it when physical conditions, like geography or climate, make a task impossible to perform. Hiking up the mountain in winter is impracticable.
Pro Tip 3/3
Not Just Hard
'Impracticable' is not just for difficult tasks, but those that can't be performed at all. Crossing the flooded river by foot is impracticable.