Hard, Soft, and Ambiguous Consonants

There are three distinct groups of consonants in Czech: HARD, SOFT, AMBIGUOUS

Hard consonants: H, CH, K, R, D, T, N

Soft consonants: Ž, Š, C, Č, Ř, Ď, Ť, Ň, J

Ambiguous consonants: B, F, L, M, P, S, V, Z

The difference between them is due to the usage of IOTA (i, í) and YPSILON (y, ý). The hard consonants can be followed only by ypsilon, the soft ones only by iota and the ambiguous by both.

Thus Czech spelling permits only hy, chy, ky, ry, dy, ty, ny is allowed in.
Similarly, only ži, ši, ci, či, ři, di, ti, ni, ji are possible.
The ambiguous consonants can be followed by iota as well as ypsilon. Examples:

bít (to beat), být (to be), mít (to have), mýt (to wash), vidět (to see), vysvětlit (to explain), veselý (joyful, adj. M sg.), veselí (adj. MA pl.), mýdlo (soap), místo (place), lepit (to glue), slepý (blind), sýr (cheese), síra (sulphur), zítra (tomorrow), jazyk (language, tongue)

If d, t, n are followed by one of the two Czech soft vowels ě or iota, these hard consonants will change into the soft consonants ď, ť, ň.
Example: děti, těšit se, něco, utíkat etc.
Remember:
The vowel ě can be written only after d, t, n and the following ambiguous consonants: b (oběd), f (fěrtoch), p (pět), v (věta), m (město). It never follows other consonants, be they hard, soft or ambiguous. This means that ě never follows the remaining ambiguous consonants l, s or z.

These general rules, however, do not apply to words of foreign origin (for example kino, cynický, chinin, riziko, tradice, titul, Cyril, dialog, universita, etc.), which will keep their original spelling and pronunciation.

The difference between hard, soft and ambiguous consonants is important not only because of spelling, but also because it affects the declension of Czech nouns and adjectives, which can be either hard or soft.