In your form design you will first have to add a subform to contain the fields you wish to hide/show either below or to the side of your dropdown list. Set this subform to invisible or hidden to start with.
Then use the action builder to apply an action to your drop-down list. To define an action for your drop-down list box, right-click on the list box and select Actions -> Add Action ... This will open the action builder dialog where you can set the condition for the dropdown.
For example to apply an action based on its value (ie YES), change the Condition so that it reads "text selected" and set the next drop-down to your value (YES). In the result box, select "Show or Hide an Object" from the list, then click on the (object) hyperlink which allows you to browse the object tree. Select the hidden subform you have just added to your form and then set visible in the next drop down. This should display the hidden subform when the value is Yes.
You will then have to add another action to handle No, but this time hiding it instead of making it visible.
