What is the best solution for this case:
There is a Story that has 8 Story Points and 5 sub-tasks. Sprint is finished but only 3 sub-tasks are completed.
Because of that we can't consider Story finished. How can we count how many Story Points were done during this sprint? 3/5*8? or 0?
When this story is moved to the next sprint would you leave completed tasks as complete in new sprint or delete them? Would you reduce Story Points (8) for this story (because 3 sub-tasks are already done)
Sub-tasks won't have any impact on the estimates because the plan and reporting around story points works at the story level only. Story is considered done and story points are burned (for the burndown report) when you complete the story (in your case, when all the sub-tasks are done). If you move the story out of the sprint without finishing it then the velocity chart will reflect that (and it is fine). The purpose of velocity chart is to show you the capacity of the team over the last few sprints and can give you an idea about how much work your team can do.
No you won't delete the story, it will stay in the project.
Thank you for replying @Ravi Sagar _Sparxsys_
So, you are saying that if only 3 of 5 sub-tasks are finished (that means Story is not complete), we consider that 0 story points (out of 8) were done at the end of the sprint, right? What's the reasoning behind that? The team has put effort in the story but couldn't finish it... Does it mean that they've done 0 Story Points? Wouldn't it hinder in determining velocity? (because they put effort but got 0 story points)