The process is complicated by two factors. Firstly, vowel length is not indicated in the spelling, so it's not possible to tell whether the first vowel in "ui" is short or long. It is short in the illative singular and thus causes simplification ("viessu" "house" → "vīssui" "into the house"), but it is long in the plural forms and does not cause any simplification ("viessūide" "into the houses"). A second complicating factor is that under some circumstances, original long "i" and "u" in unstressed syllables have shortened to "e" and "o" (denoted in grammars and dictionaries with an underdot "ẹ" and "ọ" to distinguish them). These shortened vowels do not cause simplification, but are indistinguishable from the older originally short vowels that do trigger it. These cases must simply be memorised.