Here's a way to do that:
1) Add required text box
2) Add optional text box
3) Add checkbox
4) With the checkbox selected, expand the Conditional Logic section in the right sidebar
5) Select Create Rule
6) Click on the required text box
7) Select Checked from the drop-down box at the top of the page
8) Select DONE
9) Select Create Rule
10) Click on the optional text box
11) Select Unchecked from the drop-down box at the top of the page
12) Select DONE
13) Put both text boxes in exactly the same position (only one will be visible at a time, so they won't interfere with each other)
How do i apply the above rule across, multiple options - ie, when the checkbox is ticked they must fil in the text box associated, however they must pick a minimum of one of 5 checkboxes, each with the associated textbox