What is a landform? The usual definition is that a landform is a natural feature of the solid surface of the earth. Using that definition, the four major landforms are mountains, hills, plateaus, and plains. But that's not the whole picture. Because oceans, lakes, rivers, and ponds are full of a variety of landforms, and some bodies of water are referred to as landforms themselves if they occurred naturally, because they can be found on the solid surface of the earth. For example, the oceans are a landform of water. Over 70% of the earth's surface is ocean. A lake landform is a body of water surrounded by land. To keep things straight, you usually refer to a landform along with what type of landform it is - a lake landform, an ocean landform, a mountain landform, a desert landform.
A gas, like oxygen or hydrogen, is not a landform because it is not a feature of the solid surface of the earth. A man-make lake or hill is not a landform because it was not formed naturally.
To decide if something is a landform, ask yourself three questions. Can you draw a map or point at something and say there it is, and there it has been for the last hundred or thousand or million years or so? Was it made naturally? Is it a feature of the solid surface of the earth? If you answer yes to all three questions - it's probably a landform.