Standard past auxiliary versus colloquial omission

Understanding when and how to omit the past tense auxiliary is crucial for understanding natural spoken Czech.

1. The First Person (Já, My)
In Standard Czech, you say the standard past tense using the auxiliary: (Já) jsem zapomněl (I forgot). In informal Common Czech (obecná čeština), the auxiliary is frequently dropped, as this is the default, everyday spoken register for many speakers, particularly in Bohemia. Because the auxiliary is gone, you typically use nominative personal pronouns to mark the person: Já zapomněl (I forgot). However, in fast-paced, continuous narratives where the 1st person subject is already firmly established by a preceding clause (e.g., Já tam přišel, sedl si a čekal. [I arrived there, sat down, and waited.]), native speakers do sometimes drop both the pronoun and the auxiliary in the subsequent verbs. Be careful: if you use a bare l-participle out of nowhere without a preceding clause establishing the 1st person, it will automatically be understood as the 3rd person. Similarly for the plural, My jsme zapomněli (We forgot) becomes My zapomněli. This pattern applies identically to negative verbs (e.g., Já jsem nezapomněl [I didn't forget] becomes Já nezapomněl). Alternatively, instead of dropping it entirely, speakers frequently use the phonetically reduced forms sem and sme (e.g., Já sem zapomněl [I forgot], My sme zapomněli [We forgot]). Complete omission of the auxiliary is heavily associated with Bohemian Common Czech, whereas in Moravia, speakers are more likely to retain the reduced auxiliary forms (e.g., Já sem zapomněl). Additionally, when the auxiliary is dropped, any remaining clitics simply stay in the second position (directly after the pronoun). For example, Já jsem mu to dal (I gave it to him) becomes Já mu to dal. This is especially important for reflexive clitics: Já jsem se bál (I was afraid) becomes Já se bál. Note: The 3rd person (on/ona/ono/oni) inherently takes no auxiliary in the standard past tense (On zapomněl [He forgot]), which is why this omission pattern primarily applies to the 1st person and occasionally the 2nd person plural.

2. The Second Person (Ty, Vy) - A Crucial Difference
A very common learner error is assuming you can drop the auxiliary in the second person singular too (e.g., saying *Ty viděl? [Did you see?]). This strict grammatical restriction applies specifically to the 2nd person singular; it is generally ungrammatical in mainstream Czech. Instead of dropping the auxiliary, Common Czech reduces jsi to the enclitic -s and attaches it to the first stressed word: Tys to viděl? or Viděls to? (Did you see it?). This enclitic -s is also extremely common with question words (e.g., Kdes byl? [Where were you?], Cos dělal? [What were you doing?]). Alongside the enclitic -s, speakers also frequently use si as a separate word (e.g., Ty si to viděl? [Did you see it?]). When jsi combines with the reflexive pronouns se or si, it contracts to ses and sis (e.g., Ty ses smál? or Smál ses? [Were you laughing?]). These contracted forms are the only codified standard forms and are mandatory in formal writing. In Common Czech, instead of contracting with the reflexive pronoun, you will also hear the -s attached to the subject pronoun (Tys se smál? [Were you laughing?]), which is non-standard. For the plural or formal Vy, the auxiliary is not reduced to -s. Instead, jste is typically reduced phonetically to ste in Common Czech: Vy ste to viděli? (Did you see it?), though it can also occasionally be dropped entirely (e.g., Vy to neviděli?).

3. Register and Usage
This omission is highly informal and non-standard (nespisovné). It is extremely frequent in everyday spoken Czech, text messages, and internet forums, but it is entirely unacceptable in formal writing, official correspondence, or formal public speaking. Furthermore, this omission applies strictly to the past tense indicative. Conditional auxiliaries (such as bych, bys, by) are never dropped, even in the most informal speech (e.g., Já bych zapomněl, never *Já zapomněl).