I am unable to understand, why the sounds in english are not commonly spread. I looked into the IPA on google and could not understand, why the sounds change.
e.g. chef vs chair.
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It would be a fine thing to have no irregularities in languages. But languages were not created at the drawing-board. They evolved over long periods of time. And if a word is borrowed from French as chef ( head or master of the kitchen) then it normally has the French pronunciation and that is /shef/.
It seems you have not yet discovered how many irregularities your mother tongue probably has.