How to connect form html page to my main landing page

Answer 1

Use the anchor tag <a>

Answer 2

I don't know how are you making your app so i will answer based on what i did.

Assuming form and main are two different html pages. Then you can add a form button on main page.

When button is clicked it will make get request to server for the form page and will redirect the user to a custom url where you serve the form. But you can do this only if you understand server side coding.

If it's a single html page then use display none or block or any another value to hide and show things.

I am not saying that above are the only way but i don't see any other way . Wait for others answer as well or you can google it.