May someone explain to me how to do this in the easiest way? i don't really understand. ❤

Change one to 5/5 because that is equal to one and then subtract the top numbers to get 4/5. Hope this helped.

You have to get both parts (the number and fraction) in the form of a fraction with the same denominator.  Since anything divided by itself is equal to one (3/3, 5/5, 19/19, etc) we can write the number 1 as the denominator of the fraction divided by itself.  Here's what I mean:
1 - 1/5 = 5/5 - 1/5 = (5-1)/5 = 4/5

And then for the next one:
 1 - 3/5 = 5/5 - 3/5 = =(5-3)/5 = 2/5