i/í or y/ý?
The Czech consonants can be divided into hard (h, ch, k, r, g, d, t, n), soft (ž, š, č, ř, ď, ť, ň, c, j) and ambiguous (b, f, l, m, p, s, v, z).
The soft consonants are followed by i and hard ones by y. The words with ambiguous consonants followed by y are to be memorized by the Czechs as so-called listed words. The borrowed words are exceptions.
ú or ů?
The letter ú is written always at the beginning of a word or after a preposition (for example úkol, zúčastnit se), whereas the letter ů is written inside a word stem and at the end of a word (for example dům, stůl, domů).