Extended conditions depending on product

One of the products is an online learning system. When people order this there should be an extra (and required) checkbox with additional terms and conditions. I've added a new view, just like the regular terms and conditions. Based on the product id in the cart I wanted to display the extra field. But when it's not displayed you can't buy other products, where you don't need the extra field.
How can I display the extra required field, only if a certain product is in the cart?

SOLVED:
In custom fields you can assign a product to the field. Now the custom field will only be displayed when that product is in the cart