Let’s say you wanted to hide/show a specific textbox only if a specific checkbox has been selected. You can use jQuery to do this via SharePoint Designer 2010
How can I validate a textbox using the PreSaveAction()
How do I: Create a simple form to collect data in SharePoint 2010
How can I hide/show a textbox based on a radio button selection?
How can I hide/show a textbox based on a drop-down value?
