Past transgressive

Formation:
Take the masculine singular past tense (l-participle) of a perfective verb and remove the final -l to get the stem.

For stems ending in a vowel (e.g., udělal):

• Masculine singular: add -v (udělav)
• Feminine/Neuter singular: add -vši (udělavši)
• Plural (all genders): add -vše (udělavše)

For stems ending in a consonant (e.g., přinesl, upekl):

• Masculine singular: add -∅ (přines, upek)
• Feminine/Neuter singular: add -ši (přinesši, upekši)
• Plural (all genders): add -še (přinesše, upekše)

Negation: The negative is formed simply by attaching the prefix ne- to the transgressive (e.g., neudělav).

Irregular Stems & Exceptions:

• The verb být: It is the only imperfective verb in modern Czech that forms a past transgressive: byv, byvši, byvše. These forms are almost exclusively used as auxiliary verbs with a passive participle to form the passive past transgressive (e.g., byv pozván - having been invited; byvše zrazeni - having been betrayed). Note that in older literature, you might still encounter past transgressives of other imperfective verbs (e.g., mít → měv, vidět → viděv, jít → šed).
• Verbs in -nout: They restore the -nu- suffix and take vowel endings (e.g., vytisknout → vytisknuv, vytisknuvši, vytisknuvše).
• Compounds of jít and jet: Compounds of jít use the historical stem -šed- and take consonant endings (e.g., odejít → odešed, odešedši, odešedše). In contrast, compounds of jet follow the regular vowel-stem rule (e.g., odjet → odjev, odjevši, odjevše).

Key Rules & Nuances:
1. Relative Time: It expresses anteriority (an action completed before the main verb). The main verb is usually in the past, but it can also be in the present (e.g., Přečet knihu, vrací ji do knihovny. - Having read the book, he returns it to the library) or the future. In fact, the past transgressive is the only transgressive form used to express anteriority to a future main verb today (replacing the obsolete future transgressive), though standard subordinate clauses are the normal, grammatically correct way to express this. It is only unusual because transgressives in general are archaic.

2. Reflexive Verbs: Reflexive pronouns (se/si) are retained and simply follow the transgressive form (e.g., vrátiv se - having returned).

3. Strict Subject Agreement: The transgressive must share the exact same subject as the main clause. If the subjects differ, the sentence is grammatically incorrect (a dangling modifier).

4. Punctuation: Expanded transgressive phrases (rozvité přechodníkové konstrukce) must be set off by commas from the rest of the sentence. However, unexpanded (single-word) transgressives tightly bound to the verb are typically not separated by commas (e.g., Odešel nepozdraviv. - He left without saying goodbye.).

5. Modern Usage: In contemporary Czech, this form is virtually extinct. Native speakers replace it with a subordinate clause using když, poté co (after), or jakmile (as soon as), or by coordinating two verbs with the conjunction a.