Event registration form - How to make one field appear only if another checkbox field was ticked?
If you are using the regular CiviCRM event forms (utilizing profiles) this is only possible with customisations (probably using javascript). In case you are using Drupal you could set up the integration between CiviCRM and teh webform module which allows you to define such conditions (among many other settings such as paged forms, draft saving, advanced styling...).