Why we say plural of boot is boots and why we say that plurals if foot is feet ?
It's an example of i-mutation.
In ancient West Germanic, which is the ancestor of Old English, i-initial suffixes were added to words to form the plural and other forms. Add the plural suffix -iz to the singular noun *mann and you got *manniz. The suffix vowel -i- changed the pronunciation of the root vowel -a-, giving *menniz, which became Modern English, men; man, men.
Further remnants of i-mutation are:
- Abstract nouns formed from adjectives by adding -ith: foul-filth, hale-health, long-length, slow-sloth, strong-strength, wide-width, deep-depth.
- Verbs formed from noun or adjective roots by adding -jan: doom-deem, food-feed, tale-tell, full-fill, blood-bleed, hale-heal.
- Causative verbs formed from preterites of strong verbs by adding -jan: drank-drench, lie-lay, rose-raise, sat-set, drove-drive. Fell-fell is also an example, though it's not so obvious now.
- Noun plurals in -iz: man-men, foot-feet, tooth-teeth, goose-geese, louse-lice, mouse-mice. Along with woman-women (derived from wif-man) these are the only survivors of this class, which was numerous in Old English and included such words as the ancestors of modern book, goat, and friend, which now have gone over to the -s plural.
- Comparatives in -ir: old-elder, late-latter.
I-mutation turns up in an adjective formed from a noun by adding -ish in at least one important case: English (O.E. Englisc) from the people called Angles.