kinematics - Very basic question: When to use $s=vt$, $s=1/2vt$, $s=at$ and $s=a/t^2$?

Very basic question: When to use $s=vt$, $s=\frac{1}{2}vt$, $s=at$ and $s=\frac{a}{t^2}$? What was the difference between those?