být / stát se + Instrumental

1. With "stát se" (to become) and "zůstat" (to remain): The instrumental is strictly required for nouns. You must say Stal se učitelem (He became a teacher), NEVER Stal se učitel. For adjectives with stát se, the instrumental is often used (Stal se slavným), though the nominative (Stal se slavný) is also permissible. However, with zůstat, the nominative is the standard, neutral, and most common everyday choice (Zůstal svobodný), while the instrumental (Zůstal svobodným) is considered more formal or literary.

2. With verbs of appointing and electing: The instrumental is also the standard case used for nouns after verbs like jmenovat (to appoint) and zvolit (to elect). For example, Jmenovali ho ředitelem (They appointed him director) or Zvolili ho prezidentem (They elected him president). This functions as an instrumental complement. Note that using za + accusative (e.g., Zvolili ho za prezidenta) is also a very common and standard alternative.

3. With "být" (to be): While traditional grammar distinguishes the nominative and instrumental semantically (nominative for permanent classification, instrumental for an acquired role), in everyday modern Czech, the nominative is the default for professions (Je lékařka / She is a doctor). The instrumental (Je lékařkou) primarily marks a formal or written register. However, the instrumental is still strongly preferred when "být" is in the infinitive after modal or phase verbs (e.g., Chce být inženýrem / He wants to be an engineer).

4. Standalone Adjectives: If the predicate is a true standalone adjective with a finite form of "být", it stays in the nominative (e.g., Je nemocný / He is sick). While this is strictly true for finite verbs, the instrumental can sometimes be used with infinitive phrases in formal or literary contexts (e.g., umění být užitečným / the art of being useful). Note that this rule applies to true adjectives, not substantivized adjectives (words that decline like adjectives but function as nouns, like vedoucí or cestující). Substantivized adjectives require the instrumental after stát se (e.g., Stal se vedoucím / He became a manager).

5. Evaluative Adjectives Nuance & Agreement: If a profession is modified by an adjective that evaluates the person's inherent quality or skill (e.g., dobrý, špatný), there is a strong stylistic tendency to use the nominative (e.g., On je vynikající lékař / He is an excellent doctor). While the instrumental is still grammatically correct (Je vynikajícím lékařem), the nominative is often preferred to emphasize a permanent characteristic. Crucially, if you do put the noun into the instrumental, any attached adjective must also decline into the instrumental to match it (dobrým učitelem, not dobrý učitelem).

6. The "To je..." Identity Rule: When identifying someone or something using the demonstrative pronoun to (to je...), the nominative is strictly required. You must say To je učitel (That is a teacher), never To je učitelem. This also applies to abstract concepts: you must say To je pravda (That is the truth), and using the instrumental (To je pravdou) is incorrect.

7. The Nominative Contrast: The nominative is strictly required for permanent identities, origins, and biological classifications (e.g., Delfín je savec / A dolphin is a mammal; Je Čech / He is a Czech).

8. A Foolproof Alternative: A very common and natural way to express professions without worrying about the instrumental is using pracovat jako + nominative (e.g., Pracuje jako učitel / He works as a teacher).

9. Negation: Note that negative sentences follow the exact same rules. Negation does not change the case (e.g., Není učitelem / Není učitel), so do not confuse this with the genitive of negation found in some other Slavic languages.

Note on 'Temporariness': You might hear that the instrumental is used for 'temporary' jobs. In Czech grammar, 'temporary' simply means an acquired state or role—something you weren't born as. This applies to professions even if you hold them for your entire life.