Habitual versus single completed event

English often uses the Past Simple tense for both single events ("I read a book yesterday") and habits ("I read a book every day"). Czech distinguishes these using aspectual verb pairs.

1. Jednorázový dokončený děj (Single Completed Event)
Use a perfective verb when focusing on the result of a one-time action.

• Včera jsem napsal úkol. (I wrote/finished the homework yesterday.)

2. Jednorázový průběhový děj (Single Ongoing Action)
Use an imperfective verb for a one-time action if you are focusing on the process or duration, similar to English continuous tenses.

• Včera jsem psal úkol celý večer. (I was writing homework all evening yesterday.)

3. Opakovaný děj (Repeated/Habitual Action)
Use an imperfective verb for most actions that happen regularly, often accompanied by words like každý den (every day) or často (often).

• Každý den jsem psal úkol. (I wrote/used to write homework every day.)

Note: You can also use a perfective verb for a repeated action if you want to emphasize that each instance was successfully completed (e.g., Každý den jsem napsal jeden úkol. - Every day I successfully finished one homework assignment).

4. Slovesa pohybu (Verbs of Motion)
For basic verbs of motion, Czech distinguishes between determinate (unidirectional) and indeterminate (multidirectional) verbs. Both types are imperfective. Determinate verbs are used for single unidirectional actions, while indeterminate verbs are used for repeated or habitual actions, as well as for multidirectional, round-trip, or goalless motion (even as a single event). (Note that when directional prefixes are added, they form standard perfective/imperfective pairs, typically by making the determinate verb perfective and pairing it with a derived imperfective, e.g., perfective přijít and imperfective přicházet).

• Včera jsem šel do školy. (I went to school yesterday - single unidirectional action using determinate imperfective jít.)
• Každý den jsem chodil do školy. (I walked to school every day - repeated action using indeterminate imperfective chodit.)

For strongly habitual actions, Czech also uses specific iterative verbs in -ávat and -ívat (e.g., psávat, chodívat).

• Dříve jsem psával úkoly ráno. (I used to write homework in the morning.)

These iterative verbs can also be used in the present tense (e.g., psávám), but they only describe general habits, never an action happening right at this moment.

5. Budoucí čas (Future Tense Formation)
The choice of aspect dictates how the future tense is formed:

• Imperfective verbs: Use the future tense of být (budu + infinitiv). Example: Budu psát úkol. (I will be writing homework.)
• Perfective verbs: Use the present-tense conjugation of the perfective verb (perfective present with future meaning). Example: Napíšu úkol. (I will write/finish the homework.) Never use budu with a perfective infinitive!
• Determinate motion verbs: Form the future by adding the prefix po- / pů- to the present tense form (e.g., půjdu / pojedu). Example: Půjdu do školy. (I will go to school.) Never use budu with a determinate motion infinitive (like budu jít)!

Phase Verbs: Note that phase verbs indicating the start, continuation, or end of an action (like začít or přestat) must always be followed by an imperfective infinitive (e.g., Začal jsem psát, never Začal jsem napsat).