1)All strong acids in the aqueous solution appear almost equally strong ,i.e, they ionise completely and their relative strength cannot be compared. This phenomenon is known as levelling effect. What's the cause of this ?

 

2)Flourine is the most electronegative element in the periodic table, still HF is considered to be the strong acid and it has no basic properties at all ( I read this in one book). But I have a doubt since it is electronegative shouldnt it have the tendency of attracting the proton towards itself. And why is it so strong that even the strongest acid such as nitric acid also behave like base when treated with HF ?

With respect to the HF question, the fact that fluorine is so electronegative is why it doesn't act as a base. F- is more stable than HF because the electron from hydrogen is more stable on the flourine than on the hydrogen. But remember that it is dissolved in water so the proton with be on H3O+. This will act as a counter ion to the fluoride in solution (i.e. a H3O+ will be close to a F-) which again stabilises the dissociated side of the equilibrium.

 

If the F- acts as a base, then you would have HF again, which as I've just expalined is less stable than F- and H3O+.