Aspect in infinitives after modal verbs

Unlike English, which uses the same bare infinitive for both concepts (e.g., 'I have to read'), Czech forces a choice. In affirmative sentences, modal verbs are 'transparent'—they do not dictate the aspect themselves. The choice of aspect depends primarily on the nature of the action and the speaker's intent. However, negation changes the rules and actively influences aspect choice.

1. Single vs. Repeated Actions
If you need to finish reading a specific book, use the perfective: Musím si to přečíst. If you need to spend an hour reading every day, use the imperfective: Musím číst každý den.

Rule of Thumb: If you can mentally add 'completely' or 'just once' to the English sentence, use the perfective. If you can add 'regularly' or 'for a while', use the imperfective.

2. General Abilities vs. Specific Requests
Modals like umět (to know how) and moci (to be able) often pair with imperfective infinitives for general abilities (e.g., Umím plavat - I know how to swim). However, moci frequently takes the perfective aspect for single, specific offers or requests (e.g., Můžu ti pomoct? - Can I help you?).

3. The Modal 'chtít' (to want)
The verb chtít follows the exact same rules. Use the perfective for a specific goal (Chci to udělat. - I want to do it.), and the imperfective for an ongoing process or habit (Chci to dělat každý den. - I want to do it every day.).

4. The Modal 'mít' (should / supposed to)
The modal verb mít follows the exact same logic. Use the perfective for a specific recommendation or expectation (Měl bys to udělat. - You should do it.), and the imperfective for a general rule or repeated expectation (Máš chodit včas. - You are supposed to arrive on time.).

5. The Negation Nuance
Negation actively pulls the infinitive toward specific aspects. For nemuset (not have to), Czech heavily favors the imperfective aspect to cancel the process of an action, even for single, one-off events (e.g., Nemusíš to kupovat. - You don't have to buy it [canceling the need to do it]). For nesmět (must not), using the perfective forbids the result of a specific act (Nesmíš to ztratit. - You mustn't lose it), while the imperfective forbids a general practice or engaging in the process at all (Nesmíš tady kouřit. - You mustn't smoke here).

6. The 'Failed Attempt' with 'nemoci'
Negating moci (to be able to) with a perfective infinitive often expresses an inability to achieve a result despite trying—a 'failed attempt' (e.g., Nemůžu to najít. - I can't find it). In contrast, using the imperfective means you cannot even engage in the process (e.g., Nemůžu to hledat. - I can't look for it [because I don't have time]).

Note: Do not confuse modals with phase verbs like začít (to begin). As explained in phase verbs with imperfective infinitive, phase verbs strictly require an imperfective infinitive, whereas modals are transparent and freely take either aspect.