Colloquial Present Endings

Open a textbook and you will see kupuji "I buy" and kupují "they buy." Spend a day in Prague and you will hear kupuju and kupujou. Neither is a mistake — they are two registers of the same verb, and knowing which to use when is one of the most useful pieces of sociolinguistic knowledge a learner of Czech can have. This page maps that split: where it happens, where it does not, and how to choose.

Where the split lives: only 1sg and 3pl

The double forms appear in exactly two slots of the present tense — the 1st person singular and the 3rd person plural — and only in certain conjugation classes. The verbs affected are the -je- and -ovat verbs of class III and the soft -e- verbs of class I (the píše, maže type). Everywhere else in the paradigm the two registers are identical.

Look at the table: four of the six forms are shared. The fight is only over já (-u vs -i) and oni (-ou vs -í). The verb krýt "to cover" behaves the same way — kryju / kryji and kryjou / kryjí — and so does every single -ovat verb, of which Czech has thousands.

These are not "wrong" forms

It is tempting to file kupuju under "slang to avoid in writing." That would be inaccurate. The -u/-ou endings are codified as standard colloquial Czech (hovorová spisovná čeština) — they are inside the official standard, just marked as the spoken layer of it. This makes them quite different from genuinely non-standard obecná čeština features like seš for jsi "you are." You can use kupuju in conversation, in friendly emails, in chat, and in most semi-formal speech without anyone batting an eye.

In practice the -u/-ou forms dominate everyday spoken Czech across Bohemia. The -i/-í forms, by contrast, sound bookish or carefully formal: you reach for them in a job application, a news report, an academic paper, or a prepared speech. Using děkuji "thank you" to a close friend in a casual text can even sound slightly stiff or over-polite.

Classes that do NOT split

This is where learners over-apply the pattern, so it is worth stating plainly. The -u/-ou vs -i/-í split does not exist in:

• Class V, the dělá type. Here dělám "I do" and dělají "they do" are register-neutral. There is no dělaju. (See class A.)
• Class IV, the prosí type. Prosím "I ask" and prosí "they ask" are the standard forms everywhere. Forms like prosijou exist only in Moravian dialect, not in mainstream Common Czech, and they sound regional.

So the split is a property of specific classes, not a switch you can flip on any verb. If the verb is dělat or prosit, there is simply nothing to choose.

A worked contrast: the same idea, two registers

Because only 1sg and 3pl move, you can take an entire sentence and shift its register by touching just one or two verb endings. Compare a message you might send a friend with the same thought written for an official context:

The shared forms (kupuješ, kupuje, kupujeme, kupujete) mean you never have to re-learn a whole paradigm to change register. You learn one verb twice in two places and you are done.

How this fits the bigger register picture

This vowel choice is one thread of the larger weave of standard versus colloquial Czech. It is more "respectable" than the classic obecná čeština markers (like -ej for -ý, or prothetic v- in vokno for okno), precisely because kupuju sits inside the codified standard while those others sit outside it. For a learner, the safe default is: speak with -u/-ou, write formally with -i/-í, and you will sound natural in conversation without ever being wrong on paper.

Common Mistakes

The errors fall into three buckets: confusing the 1sg with the 3pl, inventing a split in classes that don't have one, and clashing a colloquial ending with a formal context. Keep the slot (1sg vs 3pl), the class (only I and III split), and the situation (speech vs formal writing) clear, and these endings become a quiet superpower for sounding like a real speaker.