yeah, shto is related to the latin word 'est' also, but it means "why", in bulgarian they say "zashto" which basically sounds like "for why" and so "zashtoto" is "the why" because -to means "the" similar to the scheme that they also use in nordic countries with the postfix definite article, to, ta, and ut/at.

as a rule, serbian is much easier to pronounce for an english or west european speaker, compared to bulgarian, and bulgarian is like russian but faster and more staccato. russians can pretty much understand what serbs are saying without learning serbian also, and i have heard it referred to as being like "baby russian"

and yeah, that whole thing wish jest, which is written ECT in russian is the same as the latin "est" and there is some similar uses in portuguese too, it basically means "it is" and it's the same as the root of the english word "yes" and sounds very similar, yes, yest, yeste, eshte.

other words that are very close along these are "za" "fuer" "para" "por" the word "for" in english. that word "zato" means "for it" more or less literally, to is the neuter second person pronoun, what you use when refering to a child.