It's an easy to make a mistake. I will say this much: in English, using a gerund where an infinitive is needed, and vice versa, is wrong, but does NOT make it hard to understand. Your sentence is incorrect. This is one way to correct it: To find the area of a circle, multiply the square of its diameter by .785398. I don't know the formal grammar but I suggest you just practice by reading aloud and repeating other sentences on the same pattern. --To make good coffee, use one measure of ground coffee per cup. --To find Arcturus, find the Big Dipper and continue the arc of the handle: "follow the arc to Arcturus." --To get to Antartica, start anywhere and just head south. You can't miss it! Here's an example where you'd use the gerund. --When finding the area of a circle, be sure to use the right formula.