Can you completely bring any work to an end once you have started it?

The purpose of starting any job / project / assignment is to ensure that you achieve the end result, which is only possible to see, after its completion. 

Therefore, an executor of a job should have well planned outline of the job to predict its possible result.  In this way, if any unexpected obstacles arise, you can focus on that problem, as all other predictable problems are already taken care of at the planning stage.

If your question is related to know about my personal experience till date, I can safely say that till date whenever I had taken on a task, I make sure that it is completed beyond the satisfaction of my superiors.

Yes I can if I did the same task before at then I've the experience to do it in just less time

It is essential that you complete one task/project at a time before starting a new one, unless you are required for another project which higher priority.