1. 'Hair' (uncountable) is what you have on your head - it is your 'coiffure' or your 'chevelure', if you like. It can never be plural. The only time we might talk about the 'hairs' on someone's head is as a joke. If a bald man really only has about five hairs on his head, it's possible to make a joke about this by referring to them as 'hairs'. He now has individual hairs that you can count, as opposed to the full head of 'hair' that he had when he was younger. Occasionally we can use 'hairs' in the plural individual strands of hair when they aren't attached to the head, or for those elsewhere on the body (les poils). For example, 'The hairs on the back of his hand'. 2. A floor is in the interior of a building. It is a man-made feature of the construction of the building. A room inside a building has walls, a floor, and a ceiling. Ground is more general, and it is out of doors. It can be man-made (such as on a tennis court) or it can be natural ( grass, sand or earth). Soil is a substance. Plants grow in it. If you want to grow a geranium in a pot, you put soil in the pot and then plant the geranium in the soil. One of the uses of the word 'earth' is a near-synonym of soil.

Diffrence between hair and hairs here: http://www.youtube.com/watch?v=cNXjRTeBxTo

Normally, the floor is what you walk on inside a building, and the ground is what you walk on outside. I too find it jarring to hear the words “floor” and “ground” used interchangeably. But many people do this. Not only do they refer to the floor as the ground, but they call the ground the floor.