Attaching Conditional widget on Drupal Field I have extended drupal user registration form by attaching fields to user entity.

One of the field is to let user select duration. I have set it up as radio button.

Form: When you started: 

(radio button) Not yet started (need help) 

(radio button) Just starting (1-2 years) 

(radio button) Traction: (2-3 years) 

(radio button) Experienced: (4+ years) 

(radio button) We started our CI program in: (field for year)


Now the requirement is if the user select any of the top 4 options it shoudl go as it is. But if none of the above suites users choice than he will select the last radio button and on this radio button he should be able to type his year manually. So basically a text field should appear. I've no clue how to achieve this and didn't find it on google too.
You should create another field for the year and use e.g. Conditional fields module so that year field appears only when a specific option is selected.

Generally speaking, you should use form API states processing, and the Conditional fields provides UI for that.