Sudoku Players' Forum (clone)

I hope you would not mind that my English is very very poor.
Could you tell me why there is only one solution for a puzzle?
I need to know this and I hope that you can give me a hand.
Thank you.

There needn't be only one solution - it depends on how well the clues are set. In a poor puzzle, the clues could lead to ambiguity. In this case, there could be more than one solution.

In a well-set puzzle, the clues are set in such a combination of positions that there can be only one valid solution.