How are sin (pi/2) and the imaginary unit different?

They appear to be in the same place, but they are on two different graphs.

 

Sin(pi/2) is found on a plane where both the x- and the y-axis are real-numbered axes.

 

The imaginary unit is found on a complex plane, where the x-axis is real-numbered but the y-axis is an imaginary axis.

You are getting confused. They are not positioned at the same place.

 

i   X  sin(pi/2)  = i   X    1      

                           = i.

 

But  sin(pi/2) is  just  1 .       So  1    and  i  are not positioned at the same place,far from it.  I think you need to look some more at complex numbers     (they aren't really all that complex ,someone just decided to call them by that name)  and get some practise with them.