byl bych býval

The formal past conditional is built using the following structure:

[First position element] + conditional auxiliary (clitic) + byl / býval / byl býval + main verb (l-participle)

Because Czech is a pro-drop language with flexible word order, the subject is frequently omitted. When the subject or an adverb is dropped, the first participle (byl or býval) usually moves to the first position to host the clitic, leaving its original slot in the formula empty (e.g., Byl bych býval přišel or Býval bych četl, but never *Byl bych byl přišel).

• Strict Agreement: All l-participles present in the construction (whether two or three) must match the subject's gender and number. For a masculine animate subject, it is byl bych býval přišel. For a feminine subject, it becomes byla bych bývala přišla.
• Word Order and Reflexives: The conditional auxiliary (bych, bys, by, bychom, byste, by) is a clitic and must occupy the second syntactic position in the clause. If the verb is reflexive, the pronoun (se/si) goes into the clitic cluster right after the auxiliary. Note that in the second person singular, the auxiliary and reflexive pronouns fuse (bys + se = by ses; bys + si = by sis), for example: Byl by ses býval smál or Byl by sis býval hrál.
• Use with 'kdyby' (If): When this pattern is used, it is typically found in past counterfactual 'if' clauses. The conditional auxiliary fuses with the conjunction kdy to form kdybych, kdybys, kdyby, kdybychom, kdybyste, kdyby. For example: Kdybych byl býval věděl... (If I had known...). Crucially, when combined with reflexive pronouns in the second person singular, the fusion is kdybys + se = kdyby ses (e.g., Kdyby ses byl býval smál...).
• When the main verb is 'být': If the main verb itself is být (to be), standard Czech strictly avoids repeating the participle byl. The correct past conditional form is simply byl bych býval (I would have been) to avoid repetition.
• Negation: You cannot negate the auxiliary (never say *nebych). According to the Institute of the Czech Language (ÚJČ), the negative prefix ne- can be attached to the first participle (nebyl bych býval přišel), the middle participle (byl bych nebýval přišel), or the main verb (byl bych býval nepřišel), as all are valid options. While repeating byl in affirmative sentences (e.g., byl bych byl přišel) is considered ungrammatical or a pleonastic tongue-twister in standard Czech (which is exactly why býval is used instead), it is grammatically permissible in negative sentences when one participle is positive and the other is negative (e.g., byl bych nebyl přišel or nebyl bych byl přišel).
• Register and Usage: This form is highly formal, literary, and archaic-sounding. In modern spoken Czech, the past conditional is largely obsolete. Native speakers overwhelmingly prefer the present conditional (přišel bych) to express past counterfactuals, relying on context. As a learner, you only need to recognize this pattern for reading literature or formal texts; you do not need to worry about actively producing it in conversation. Note that in spoken Czech, you might occasionally hear the non-standard colloquial bysme instead of bychom (e.g., byli bysme bývali přišli), creating a stylistic clash between the archaic structure and the colloquial clitic.
• Grammatical Classification: While older textbooks sometimes call this a "double past conditional," the Institute of the Czech Language (ÚJČ) clarifies that it is simply an intensified or extended past conditional, used to mark a more remote past or a definitively closed possibility.