It sounds like you want to be using Conditional Formatting on the drop-down list box to make it hide based on the value of the check box.

One thing to keep in mind is that if you are using managed code instead of script, the conditional formatting will not stay on the drop-down list box, and therefore you will need to put the list box in a section and place the conditional formatting on the section. I believe script forms do not have this issue.

hello, I am blocked on a form with a problem difficult to explain
I have a  item that I want to present under a drop-down list but also as a
boolean.
I try to explain : on this item if in the case option is yes I can choose in
the drop-down list but if it's no the drop-down list which is in a section
disappears ....
I hope you understand my problem.
How I can present this in a correct schema xsd ????