Placing se and si in the Past and Conditional

Czech parks its little unstressed words — clitics — in second position in the clause, and lines them up in a fixed internal order. The past tense and the conditional are exactly where this gets tricky, because each of them brings its own clitic auxiliary (jsem, jsi… for the past; bych, bys, by… for the conditional), and that auxiliary now has to share the slot with reflexive se or si. This page is a zoom-in on that one high-frequency collision. Get the order right here and you've cracked the most common two-clitic meeting in everyday Czech.

The one rule: auxiliary first, then se/si

When the past or conditional auxiliary meets the reflexive, the auxiliary comes first and se/si follows it. The whole pair then sits together in second position.

So in the past tense:

And when a pronoun object joins, it slots in after se/si, never before:

The cluster here is jsem (slot 1) + si (slot 2) + to (slot 4) — jsem si to, in that order and no other.

The conditional works the same way

Swap the past auxiliary jsem for the conditional auxiliary bych and nothing about the ordering changes: the auxiliary leads, the reflexive follows.

The fusion forms: ses and sis

There's one twist, and it's in the second person singular. The 2sg past auxiliary is jsi and the 2sg conditional auxiliary is bys — and when either meets the reflexive, the -s fuses onto the reflexive to give ses (from jsi/bys + se) and sis (from jsi/bys + si). Standard Czech requires the fused form; the unfused "umyl jsi se" is considered substandard.

• past: jsi
  • se → ses ; jsi
    • si → sis
• conditional: bys
  • se → by ses ; bys
    • si → by sis

The 3rd person has no auxiliary — so se moves up

This is the trap that produces the most wrong sentences. The past tense has no auxiliary in the third person (no jsem/jsi equivalent for on/ona/ono/oni). With the auxiliary gone, there's nothing for se to follow — so se itself takes the second-position slot, right after the first stressed element.

The mismatch English speakers fall into: hearing Umyl jsem se ("I washed") and assuming the jsem is part of the verb, then reusing it for "he washed." But jsem is the first-person auxiliary. "He washed" has no auxiliary at all — just Umyl se.

The whole block still obeys second position

None of this is glued to the verb. The auxiliary-plus-reflexive cluster occupies the second slot of the clause, which means if you front some other element — an adverb, an object, the subject pronoun — the cluster jumps to right after it, and the participle is left wherever it lands.

In Včera jsem se nevyspal, the adverb včera takes first position and the cluster jsem se follows it; the participle nevyspal comes later. The same machinery governs every clitic in the clause — see clitic second position for the foundation and clitic chain order for the full ordering of longer chains.

Common mistakes

The headline error is attaching se directly to the verb and putting it ahead of the auxiliary:

The second is keeping the 1st-person auxiliary for a 3rd-person subject:

The third is failing to fuse in the 2nd person singular:

The fourth is starting a clause with the auxiliary, which a clitic can never do:

Key takeaways

• When the past/conditional auxiliary meets the reflexive, the order is auxiliary + se/si (jsem se, bych se, jsem si to).
• The 2sg auxiliary fuses with the reflexive: jsi/bys
  • se → ses, + si → sis (conditional: by ses, by sis).
• The 3rd person past has no auxiliary, so se alone fills second position: On se vrátil, not "on se jsem vrátil."
• The whole cluster still lives in second position; fronting any element pulls jsem se / bych si to right after it.