Basic aspect pairs

Aspect is the core operating system of the Czech verb. You must choose the aspect before you apply past, present, or future tense rules. A key practical difference is that imperfective verbs are typically used for ongoing, continuous, or in-progress actions, as well as habitual, repeated, or routine actions, even if they are completed, whereas perfective verbs are used for single, one-time completed events.

The Past Tense: In the past tense, both perfective and imperfective verbs use the exact same grammatical structure (the l-participle plus the auxiliary verb být, such as jsem, jsi, jsme). Crucially, the l-participle must change its ending to agree with the subject's gender and number (e.g., dělal for masculine, dělala for feminine, dělalo for neuter, and distinct plural endings like dělali for masculine animate, dělaly for feminine and masculine inanimate, and dělala for neuter). Note that the auxiliary verb is strictly omitted in the 3rd person singular and plural (e.g., on dělal, not on je dělal). The only difference is the meaning conveyed by the aspect.

How pairs are formed:
There is no single universal rule, but pairs are usually formed in three main ways:

• Prefixation: Adding a prefix to an imperfective base creates a perfective verb (e.g., psát (to write) → napsat, číst (to read) → přečíst, dělat (to do/make) → udělat).
• Suffixation: Altering the suffix of a perfective base creates an imperfective verb (e.g., koupit (to buy) → kupovat). It also works in the opposite direction to create perfective verbs for sudden, single actions, typically using the suffix -nout (e.g., kopat (to kick/dig) → kopnout).
• Suppletion: Using completely different roots (e.g., brát (to take) → vzít).

Additionally, when a prefix changes the meaning of a verb, a new suffix is often added to make it imperfective again (secondary imperfectivization, e.g., psát (to write) → podepsat (to sign) → podepisovat).

The Present and Future Tense Nuance:
Because a perfective verb describes a completed whole, it logically cannot be happening 'right now'. Therefore, perfective verbs do not have a true present tense. This means that only imperfective verbs can be used to describe an action happening right now in the present tense. When you conjugate a perfective verb in the present tense forms, it automatically creates a future meaning (e.g., napíšu means 'I will write/finish').

Conversely, to talk about the future with most imperfective verbs, you use the analytical future (budu + imperfective infinitive, e.g., budu psát - I will be writing). A major exception is determinate verbs of motion, which form their future with a prefix (e.g., půjdu, pojedu - I will go [by foot/by vehicle]). This rule also applies to other determinate motion verbs (e.g., letět (to fly) → poletím, nést (to carry) → ponesu, běžet (to run) → poběžím). Note that despite using a prefix for the future tense, these verbs remain imperfective in meaning.

The Imperative Mood: Aspect plays a crucial role in commands. Affirmative commands for a single action usually default to the perfective aspect (e.g., Kup to! - Buy it!). However, negative commands (prohibitions) almost always require the imperfective aspect, even for a single event (e.g., Nekupuj to! - Don't buy it!).

Bi-aspectual verbs: Some verbs, especially those borrowed from other languages (e.g., informovat (to inform), organizovat (to organize)), are bi-aspectual (obouvidová slovesa). This means the same form handles both perfective and imperfective functions depending on the context. However, modern Czech often adds prefixes to these borrowed verbs to make them unambiguously perfective (e.g., organizovat → zorganizovat (to organize), informovat → zainformovat (to inform)).