what's the best practice to divide long task into functions? suppose I have function of 200+ lines then from the code readability point doing short functions would be better?

You might want to read about Clean Code - it contains a lot of useful methods to deal with that.

In your case, try to find blocks of code in that monster function that do only one thing and move them in a different function or in a separate class - if you have related stuff.

It's simple, really.
Piece of code used twice in the same class?
-make a function.

Piece of code used in 3 or more classes?
-make a static class with static functions.

Piece of code used in more classes and there's no way to know for sure if values are being given to certain variables?
-make a singleton class that includes functions and variables.

Cannot read code?
-make regions (c#).

I simply use something like that to make it through.

Single responsibility rule. Chances are your function is doing a gazillion things now. extract functions so hopefully they only do one thing or at most two.
https://sourcemaking.com/refactorin...