Fit can be used in so MANY different ways. In most cases, it is used as an adjective. Fit - to be the right shape/size/number of Because I am fit, I will be able to go through the door (Because I am this size, I can fit through the door) It is uncommon/rare to see it as a noun by itself, but it does have a noun form fitness - the process of getting fit Ex: I read a book on fitness Lastly, you CAN use it as a verb, but also not common. Ex: I was trying to fit myself into the t-shirt, but it did not work. (I was trying to be (the right shape) of the shirt, but I am not the right shape)

Majed, to help you keep your dictionary skills up to date and to stimulate your ability to do independent research, I would like to point out that the word "fit" itself can act as a noun. Would you like to look up a dictionary and explain to us how?