The “i” instead of “ie” is actually something shared with Ukrainian vs Russian.  Look at Niet/Nyet (Нет) vs “Ni” (Ні).  Usually, though, in Ukrainian the “i” is where the “o” should be in many Slavic languages, such as “rokiw” (років) vs “rokow” (роков) or “sil'” (сіль) instead of “sol'” (соль).  The past tense of Ukrainian (in the masculine) is identical to Slovak, with “ja behaw” (я бегав) “I ran” being a good example.  It’s a little confusing, but it makes as much sense as it can, though it’s easier when the past tense ends with an “l” or a “ł”.