give account onHI is stronger acid than HF

The size of the atom on which the negative charge rests effects the stability. As a general rule, negative charges prefer to rest on larger atoms, as the charge can spread over a much larger region of space (making it more stable) than when the charge is localized in a much smaller space on a smaller atom.
Generally, this preference for placing the charge on a larger atom trumps electronegativity considerations. For example, HI is more acidic than HF, even though based on the last argument about electronegativity you might suspect that F- would be more stable than I- because fluorine is more electronegative than iodine (and thus HF would be a stronger acid than HI). This turns out not to be the case. Iodine is so much bigger than fluorine that the charge is more stable on this larger atom. Thus, the trends for acidity of the hydrohalic acids are as follows: HI is strongest, followed by HBr, HCl, and finally HF.