I’ve been trying to learn a new language (Vietnamese) and a thing that has been driving me crazy are all these instances of letters being randomly pronounced differently in different words sometimes. If you don’t think about it too much, it’s easy to go “this language is dumb, why do they do this?” But then I think about English and we have so many examples of this or other linguistic oddities that make no sense but which I’ve just accepted since I learned them so long ago.

So I wanted to generalize my question: For all the languages where this applies, why are there these cases where letters have inconsistent pronunciations? For cases where it sounds like another letter, why not just use that one? For cases where the letter or combination of letters creates a new sound not already covered by existing letters, why not make a new one? How did this happen? What is the history? Is there linguistic logic to it beyond these being quirks of how the languages historically developed?