If i write this code

x=[1,2,3,4,[18]]
y=x
x[4][0]=15
x[1]=10
print(y)

then it outputs

[1, 10, 3, 4,[15]]

but if write like this

x=[1,2,3,4,[18]]
y=list(x)
x[4][0]=15
x[1]=10
print(y)

then it outputs

[1, 2, 3, 4, [15]]

Why is this happening?

list() only creates a shallow copy of the original list, i.e. it only copies the references. For example, when you write:

z = [18]
x = [1, 2, 3, 4, z]
y = list(x)

x and y are different lists, but they still refer the same mutable object z. So by modifying x[4] of y[4], you update the same object z. On the other hand, x[4] = [15] will have no effect on y, because you're replacing the list element.