If a person falls down, the sound that they make when they hit the ground or the floor might be "boom" or "thud" depending on the size of the person and the severity of the fall. Boom is a sound that we usually think of when there is an explosion, i.e. fireworks, bombs, etc. We can use "oops" if we fall down or run into something clumsily. We can also use "oops" if we do something unintentional to someone. For example, if I am walking down the hallway and I accidentally bump into you and your coffee spills, I might say "oops, I'm sorry!". Also, we use "oops" if we forgot to do something. For example, if I asked my husband to bring milk home when he comes from work and he forgets, he might answer "oops" when I ask him where the milk is. A "tap" is softer than a "knock". Examples: Please tap lightly on the door so we don't wake the baby. I knocked loudly several times, but no one answered the door. Hope this helps.

Knocking on a door is firm and usually very loud. A tap is usually just gentle tap like a shoulder tap are you gonna knock on someones shoulder?

Okay this depends on the object. If a rock falls to the ground it will thud. If a meteorite falls on to earth it will explode which is boom.