why sine zero is zero and cos zero is one?

Imagine a circle on a graph where the radius is always equal to 1.

On any point of that circle, which we mark with angle measurements going around the circle, your sine is going to be the y value at that point and your cosine is going to be the x value at that point.

For example, at the angle 0, your point is on the right side of the x axis, and since the point is 1 unit away from the origin, your x value is equal to 1 and your y value is equal to 0. Therefore cos(0)=1 and sin(0) = 0.

At 90 degrees, your point is on the y axis.

your y value is 1 because we're on the y axis and the point is always 1 away from the origin.

Your x value is 0.

Cos(90) = 0 Sin(90)=1

At 45 degrees, your point is still 1 away from the origin, but not on the x axis or the y, but somewhere in between.

At that point your x value happens to be square root of 2 over 2

your y value happens to be square root of 2 over 2 as well.

Cos(45) = (2^1/2)/2

sin(45) = (2^1/2)/2

Sin is the opposite side of a triangle. If an angles is zero, then the opposite side does not exist, or is 0.

Cos on the other hand, the ratio of the adjacent side over the hypotenuse. The adjacent side, with an angle of 0 is lying flat on the hypotenuse, and is the same length. Any value over itself is 1.