jsem / bych

Understanding the clitic cluster is essential to become proficient in natural Czech sentence structure.

• Placement: The cluster goes after the first logical block of the sentence. This can be one word (Včera... - 'Yesterday...') or a whole phrase (Ten nový film... - 'That new movie...'). It does NOT have to attach to the main verb.
• The First Slot: The auxiliaries (jsem, jsi, jsme, jste and bych, bys, by, bychom, byste) almost always lead the cluster. You must say jsem se, never se jsem.
• 3rd Person 'Zero-Auxiliary' in Past Tense: In the 3rd person past tense, there is no auxiliary verb. Therefore, the reflexive pronoun or other clitics (short, unstressed pronouns like mi, ti, mu, ho, mě, tě, to) move up to lead the cluster (e.g., Včera se vrátil. - 'He returned yesterday.', not Včera je se vrátil.). Crucially, this does not apply to the conditional: the 3rd person conditional DOES have an auxiliary (by), which must never be dropped (e.g., Včera by se vrátil. - 'He would have returned yesterday.').
• 2nd Person Singular Forms: In the 2nd person singular, the auxiliary combines with the reflexive pronouns se and si.
  • Past: jsi + se → ses | jsi + si → sis (e.g., Kdy ses vrátil? - 'When did you return?')
  • Conditional: The -s ending detaches from by and attaches to the reflexive pronoun, resulting in two words: bys + se → by ses | bys + si → by sis.
  Writing jsi se is considered clumsy and incorrect in standard Czech.
• Negation: The negative prefix ne- attaches to the main verb or participle, never to the auxiliary clitics (e.g., Neudělal bych to. - 'I wouldn't do it.'). The conditional auxiliary (bych, bys, by...) can never be negated (it is always neudělal bych, never nebych udělal). Note that while the past auxiliary jsem cannot be negated, the main verb jsem (meaning 'I am') CAN be negated as nejsem. Crucially, in the past tense of the verb 'to be' (být), the negation must fall on the participle (e.g., nebyl jsem - 'I was not'), never on the auxiliary (e.g., never nejsem byl).
• Conjunctions: Subordinating conjunctions (like že, když, protože) count as the 'first logical block', meaning the clitic cluster usually attaches directly to them (e.g., ...že se mu to líbí. - '...that he likes it.'). Additionally, conditional clitics fuse directly into certain conjunctions (aby + kondicionál, kdyby + kondicionál), forming the full paradigms: abych, abys, aby, abychom, abyste and kdybych, kdybys, kdyby, kdybychom, kdybyste. Crucially, because these words already contain the auxiliary, you must never add another one (e.g., never say kdybych bych or abych jsem). When these fused conjunctions in the 2nd person singular (abys, kdybys) collide with reflexive pronouns (se, si), the -s detaches from the conjunction just like it does from bys: kdybys + se → kdyby ses; abys + si → aby sis.

Note: In casual spoken Czech, you will often hear bysme instead of the standard bychom, but the cluster placement rules remain exactly the same. However, be careful never to write or say by jsme or by jste—these are extremely common mistakes, but they are strictly incorrect. The only correct forms are bychom and byste.