A complex number has two parts and they never mix. These free printable worksheets add, subtract and multiply them, and work the powers of i, with every part a whole number and every answer written as a + bi.
If they get stuck
The answer key prints on a separate page.
Adding is two sums: the real parts, then the imaginary ones. It works because i behaves like any other symbol here — three of them plus two of them is five of them — and saying that out loud is what stops a student trying to combine 3 and 2i.
Multiplying is ordinary bracket expansion up to the last term, and then one thing happens that has no counterpart in anything before it: i² is not a symbol to carry, it is −1, and a term that looked imaginary turns real and joins the other side. Every answer key gives that step a line of its own, because every wrong answer on this skill is that step missed or its sign dropped.
One question from a sheet like this one, worked a step at a time — with a sentence saying what just happened and why.
Complex numbers, step by step