Consonant Alternations in the Present

You learn the infinitive psát "to write," you reach for the present tense, and the natural thing is to say psám. It is also wrong. The real form is píšu — a different vowel and a different final consonant. This page is about that gap: a family of verbs whose present-tense stem does not look like their infinitive, because attaching the present endings forces the final consonant to soften. Once you see the pattern, these stop being random exceptions and become a small, predictable system.

What is actually happening

The present endings of the -e- and -í- classes begin with a front vowel. Many centuries ago, a front vowel (or a lost -j-) reaching back to touch a "hard" consonant pulled it forward in the mouth and changed it. Linguists call this palatalization (or, for the -j- cases, iotation). Modern Czech no longer makes the sound change live — but it froze the results into the verb forms, so the consonant you see in the infinitive is not the consonant you see in the present.

This is the single biggest reason the conjugation classes warn you never to guess the present from the infinitive. For these verbs you simply have to know the present stem, and the safest forms to memorize are the 1st person singular (píšu) and the 3rd person singular (píše), because everything else follows from them.

The core table of swaps

Here are the changes that actually surface in the present tense, with a verifiable verb for each:

The sibilants: s → š and z → ž

The psát and mazat type belong to the maže sub-pattern of class I. Their infinitive ends in a plain -at, but the present stem softens the consonant and often changes the vowel:

The same z → ž swap runs through a whole cluster of common verbs: vázat → vážu "to tie," kázat → kážu "to preach," lízat → lížu "to lick." The s → š swap shows up in česat → češu "to comb." Learn one and you have effectively learned the group.

The velars: k → č and h → ž

The "back" consonants k and h are the most dramatic. In péct "to bake" the infinitive k never appears in the present at all — every present form has č: peču, pečeš, peče, pečeme, pečete, pečou. In moci "to be able to," the h of the older stem turns to ž: můžu, můžeš, může, můžou.

The infinitive trap: -ct and -ci verbs

A handful of verbs hide their present stem behind an unusual infinitive ending in -ct or -ci. The clearest is říct (also written říci) "to say." Its infinitive looks nothing like its present, which keeps a plain k: řeknu, řekneš, řekne, řekneme, řeknete, řeknou. Notice this verb does not soften in the present — here the oddity is the infinitive, not the personal forms.

The verb chtít "to want" shows the rarer t → c swap in its present (chci, chceš, chce), which is worth flagging because chtít is one of the highest-frequency verbs in the language.

Why bother learning the system

You could memorize each of these verbs as a one-off, but you would be missing how deep the pattern runs. The very same swaps reappear all over Czech: in noun declension (ruka → v ruce "hand → in the hand," kniha → v knize "book → in the book," with k → c and h → z), in the vocative (Bůh → Bože! "God → o God"), and in comparatives (drahý → dražší "expensive → more expensive," h → ž). The verb is just one place where an ancient sound law left its fingerprints. If you treat consonant softening as a single phenomenon — covered more fully in the palatalization reference — you will start to expect it, instead of being ambushed by it.

There is no shortcut that lets you derive the present purely from spelling, though. For these verbs you must store the present stem in memory alongside the infinitive. The good news is that the set is small and the verbs are frequent, so you meet them constantly.

Common Mistakes

The thread tying all of these together is the same: the infinitive shows you the old consonant, and the present shows you the softened one. When in doubt, do not invent a present from the infinitive — recall the 1sg and 3sg you have heard, and build the rest of the paradigm from there.