Form logic

I have a form for teachers to upload documents to share with their colleagues ([login to view URL]), which is processed on another page ([login to view URL]). Two of the fields ask the teacher to choose a subject and a topic (both drop down menus). The selections are based on uploads already in the table uploads (sql dump attached). What I'd like is this logic applied to the form: - I'd like The word "Topic" and the 'topic' dropdown list to remain hidden until a subject is selected. - In the subject options is an option to Add new subject. If this is selected, I'd like the "new_subject" text box to appear as well as the word "Topic" and the "new_topic" textbox. - If an existing subject is chosen, then the "new_subject" text box should remain hidden, but the word "Topic" and the "topic" dropdown list should appear. The topic dropdown list should only contain topics which correspond to the subject selected from the 'subject' dropdown list. Eg: If the user selects the subject "Maths", then only the topics which are associated with the subject Maths should appear in the 'topic' dropdown list. - If the user selects "Add new topic" from the topic dropdown list, then the "new_topic" text box should appear. Whether the user selects an existing subject ('subject' dropwdown) or adds a new one ('new_subject' textbox), this information should end up in the subject field of the uploads table. Whether the user selects an existing topic('topic' dropwdown) or adds a new one ('new_topic' textbox), this information should end up in the topic field of the uploads table.