For the purposes of the ACT, you should remember the pattern for imaginary numbers: i = i, i2 = -1, i3 = -i, i4 = 1, then the pattern continues to repeat.
Example: What is i23? (Find the closest multiple of 4 that is less than 23. It is 20. i20=i4. Then count up to 23: i21=i1, i22=i2, and finally, i20=i3, which equals -i, so i23 = -i.