Ok, let's make one thing clear. Forms on web pages have got nothing to do with php, it is HTML.
To understand it and to be able to do what you want I suggest that you learn how to code HTML without any special tools. When you know that you can use different tools and just change what you need in the code. If you don't know the basics first you will only get in the trouble you are in now, don't know enough to correct the auto-generated things.
To be able to show and hide things on the page depending on selections is done with JavaScript. When you have learned the structure and basics of HTML you might want to learn this as well.