I assigned the 24 hour calendar to some of the activities. before assigning, I had entered their duration when they had standard calendar. but now for example even if it says 2 day duration, it shows the start and finish the same day, because of 24 hour calendar so it doesnt make sense. how can I show durations of those activities properly? so when i say 2 days for that, I mean 2 days based on 24 hour calendar now. i also tried entering hours to work column but when i do that nothing change for duration

You can only define the definition of a "day" once per project file. My suggestion is to assign resources and specify their work. 24 hours of work on the standard calendar will take 3 working days from beginning to end. The same task assigned the 24 hours calendar will start and finish in a 24 hour period.