Predicate instrumental

The predicate instrumental is used to express roles, professions, or states, especially after specific verbs of change or appointment. As a quick reminder, the most common singular instrumental endings are -em for masculine and neuter nouns, and -ou (hard) or -í (soft, e.g., kolegyně -> kolegyní) for feminine nouns, while adjectives typically take -ým or -ím (masculine/neuter) and -ou or -í (feminine). Note that some professions are grammatically adjectives acting as nouns (e.g., vedoucí - manager, cestující - passenger) and take standard adjectival endings (e.g., Stal se vedoucím - He became a manager).

• After verbs of becoming: After verbs like stát se / stávat se (to become), the instrumental case is the required standard form for nouns (e.g., Stal se učitelem). For adjectives, both the instrumental and the nominative are permitted (e.g., Stal se slavným or Stal se slavný). These rules apply equally to negative verb forms (e.g., Nestal se učitelem - He did not become a teacher).
• Verbs of appointing and electing: For verbs like jmenovat (to appoint) and zvolit (to elect), you can use either the predicate instrumental or the construction za + accusative. Both are fully standard and common alternatives. In the active voice, these verbs require an accusative object alongside the role: Zvolili ho [acc] prezidentem [inst] or Zvolili ho za prezidenta (They elected him president). These verbs are also frequently used in passive constructions where the person becomes the nominative subject (e.g., Byl zvolen prezidentem or Byl zvolen za prezidenta - He was elected president).
• After the verb zůstat: After the verb zůstat / zůstávat (to remain), the instrumental is strictly required for nouns assigning a role or profession (e.g., Zůstal učitelem). For adjectives after zůstat, the nominative case is overwhelmingly used (e.g., Zůstal zdravý - He remained healthy).
• Verbs of seeming/appearing: After verbs like zdát se (to seem) and jevit se (to appear), the rules depend on the word type. For nouns, the infinitive být must be inserted before the instrumental (e.g., Zdál se být dobrým člověkem - He seemed to be a good person). For adjectives, the nominative is the primary standard choice (e.g., Zdál se unavený - He seemed tired), while the instrumental is bookish or requires the infinitive být (e.g., Zdál se být unaveným).
• Context-dependent: After the verb být (to be) - see below.

No Preposition
Remember that the predicate instrumental is used without any preposition. English speakers often mistakenly add the preposition s (with), but you should never say Stal se s učitelem. It is simply Stal se učitelem.

The verb "být" (to be)
In the present tense, using the instrumental for professions (e.g., Je učitelem - He is a teacher) sounds formal or bookish. In everyday speech, native speakers overwhelmingly prefer the nominative (Je učitel - He is a teacher) or the construction jako + nominative (e.g., Pracuje jako učitel - He works as a teacher). However, in the past tense, the instrumental is very natural and common for describing past roles or phases of life (e.g., Celý život byl vojákem - He was a soldier his whole life). Crucially, this flexibility only applies to nouns. Standalone adjectives after the verb být must always take the nominative case (e.g., Je slavný - He is famous), never the instrumental (never say Je slavným).

Nominative vs. Instrumental
Traditional grammar often explains this as a difference between "permanent" (nominative) and "temporary" (instrumental) states. While this is a helpful starting point, a highly effective way to understand it is classification vs. function. The nominative classifies who someone is (inherent identity), while the instrumental emphasizes an assigned role, capacity, or a changed state. Crucially, for pure identity or pointing things out (e.g., To je můj bratr - This is my brother), you must use the nominative, never the instrumental (never say To je mým bratrem).