Past conditional

The past conditional (podmiňovací způsob minulý) expresses a counterfactual or unrealized past event—situations involving regrets, alternative histories, or unfulfilled conditions.

Formation:
It is a compound structure that typically stacks two l-participles:

• l-participle of být (byl, byla, bylo, byli...)
• conditional auxiliary (bych, bys, by, bychom, byste, by)
• l-participle of the main verb (udělal, šla, viděli...)

Example: byl bych udělal (I would have done). Both l-participles must agree in gender and number with the subject.

Important Formation Rules:

• The verb "být": In affirmative sentences, Czech forbids repeating "byl" (e.g., "byl bych byl" is ungrammatical). If the main verb is "být", the second l-participle must be "býval" (e.g., byl bych býval). However, in negative sentences, repeating the l-participle is grammatically correct (e.g., nebyl bych byl or byl bych nebyl).
• Three l-participle variant: An occasional, emphatic variant used to stress a more distant past uses three l-participles by adding "býval" (e.g., byl bych býval udělal). All three participles must agree in gender and number (e.g., a female speaker must say byla bych bývala udělala).
• Negation: To form the negative, the "ne-" prefix can attach to either the auxiliary participle (e.g., nebyl bych udělal) or the main verb's participle (e.g., byl bych neudělal). Both are grammatically correct.

Register and Usage:
According to the Czech Academy of Sciences, this form is highly formal and literary. In contemporary everyday Czech, native speakers overwhelmingly use the present conditional to express past counterfactuals, relying on context (like "yesterday" or a past-tense condition) to anchor the meaning in the past. When the past conditional is used today, its primary functional use is for disambiguation when the past time frame is not clear from the context. Furthermore, in spoken Czech, the 1st-person plural auxiliary bychom is almost universally replaced by the colloquial bysme (standard bychom vs. non-standard bysme). Using this in the past conditional (e.g., byli bysme to udělali) creates a noticeable stylistic clash, mixing a highly formal grammatical structure with a colloquial element.

Other Rules:

• Word Order: Conditional auxiliaries (bych, bys, by...) are clitics and must strictly occupy the second position in the clause (Wackernagel's position). For example: Byl bych to udělal vs. Já bych to byl udělal.
• Reflexive Fusion: In the 2nd-person singular with reflexive verbs, the -s ending detaches from the auxiliary bys and attaches to the reflexive pronouns se/si, resulting in by ses and by sis (e.g., byl by ses, never byl bys se).
• Conjunctions: In subordinate clauses, the auxiliary fuses with the words kdy and aby to form kdybych, kdybys, kdyby... (e.g., kdybych byl věděl) and abych, abys, aby, abychom, abyste.