Adding a formula dynamically to an excel cell

Is there a way (VB?) to add a formula dynamically to an excel cell? I have a cell with conditions true and false, and they change based on a checkbox. Now I have another cell, which has a formula in it. This formula should be used if the checkbox is unchecked. If the checkbox is checked, then user should be able insert value manually (without any formula prompting there). So the formula should not be there. I was thinking of a solution where I would add the formula to the cell if checkbox is unchecked, and then if the checkbox is checked, then I would clear the cell. How could this be done? I'm not very familiar with excel coding and VBA.