When a prefix ending with a vowel is attached to ísť, the initial í- changes to j- and the final vowel often becomes long or diphthongised, e.g. in nejsť (the negative form), prejsť(“to cross”), zájsť(“to go”), dôjsť(“to arrive”), etc. If the prefix ends with -i, the initial vowel is completely assimilated: prísť(“to come”) (pri- + ísť).
The l-participle (used for forming the past and the conditional) has two completely interchangeable forms: 1) šiel, šla, šlo, šli or 2) išiel, išla, išlo, išli. Compounds of ísť use the shorter stem with the short form of the prefix (e.g., došiel < dôjsť(“to arrive”)).
“ísť”, in Slovníkový portál Jazykovedného ústavu Ľ. Štúra SAV [Dictionary portal of the Ľ. Štúr Institute of Linguistics, Slovak Academy of Science] (in Slovak), https://slovnik.juls.savba.sk, 2003–2024