If you check your standing orders, it will explain how matters can be debated and resolved but usually a proposal is made and debated. If there are any amendments to that proposal, they are voted on first before the substantive proposal is voted upon. However, there is a world of difference between an amendment and a fresh proposal so an amendment cannot, for example, negate the original proposal (as that would be achieved by simply voting against). An amendment is just that; an amendment and not a fresh proposal which might need a entirely new agenda item if it is substantially different from the original.