Concentration solutions

can you explain this to me please,
the hydrogen ion concentration of a solution prepared diluting 50.0ml of 0,10M HCl with water to 500.0ml of solution is….
the answer is 0,010M

You go from 50 ml to 500 ml. It means you dilute 10 times.
So, your HCl koncentration will be diluted 10 times too = 0,1M/10=0,010 M.

this is the hcl concentration diluited yes, but the hydrogen ion concentration is the same?

No. It is not the same, because you have diluted 10 times. So, your H+ concentration has also been diluted 10 time. Remember, concentration is measured as moles per volume units. When you changed your volume, it means you changed your concentration (M or Molarity) too.

Number of H+ ions in the solution before and after the change is still the same.

[HCl] is the same as [H+] in a particular volume.