Spelling the Conditional Auxiliary | Czech Grammar

The conditional auxiliary is tiny, but spelling it correctly is one of the genuine shibboleths of educated Czech — and one that even native speakers trip over. There are exactly six forms to know, three fused variants, and one obligatory contraction with the reflexive. Get them right and your written Czech instantly looks careful; get them wrong and you reproduce the very mistakes that schoolteachers spend years stamping out. This page is the spelling reference, separate from the meaning (see forming the conditional) and the placement (see the word order of bych).

The six standalone forms

Memorize these as a block. They never take a -jsi or -jsme ending — they are their own complete words.

Person	Standalone auxiliary
já (I)	bych
ty (you, informal sg)	bys
on / ona / ono	by
my (we)	bychom
vy (you, pl/formal)	byste
oni / ony / ona	by

Kdybys chtěl, mohl bys zůstat.

If you wanted, you could stay. (to a man)

Měli bychom jít, už je pozdě.

We should get going, it's late already.

Přál bych si trochu klidu.

I'd wish for a bit of peace. (male speaker)

The two danger spots are bys (2nd singular) and bychom (1st plural) — the two forms English speakers most often mangle by analogy with the present-tense auxiliary jsi and jsme. Drill those two specifically.

The cardinal errors: by jsi and by jsme

Here is the heart of the matter. Czech learners — and a depressing number of natives — reason that since the present-tense auxiliary is jsi (you) and jsme (we), the conditional must be by jsi and by jsme. It must not. Those are flat errors in standard Czech.

Meaning	WRONG	CORRECT
you would (sg)	❌ by jsi	✅ bys
we would	❌ by jsme	✅ bychom
you would (pl)	❌ by jste	✅ byste

❌ Co by jsi dělal?

Incorrect — the 2sg conditional is one word, bys.

✅ Co bys dělal?

What would you do? (to a man)

❌ Měli by jsme jít.

Incorrect — the 1pl conditional is bychom, not 'by jsme'.

✅ Měli bychom jít.

We should go.

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The whole thing collapses to a slogan: bys, not *by jsi; bychom, not *by jsme; byste, not *by jste. If you ever see a space inside the auxiliary in your own writing, you have almost certainly made the mistake.

bysme is colloquial — recognize it, don't write it

In casual speech you will constantly hear bysme for bychom ("we would"). It is a real spoken form, not the same blunder as by jsme — it is a single word, and it is extremely widespread. But it is colloquial (informal): it belongs in chat, texting, and relaxed conversation, never in formal writing, exams, or careful prose.

Bysme měli vyrazit. (informal/spoken)

We should head out. (colloquial for 'Měli bychom')

Měli bychom vyrazit. (standard)

We should head out.

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Hierarchy for "we would": bychom (standard, always safe) > bysme (colloquial, spoken only) > ❌ by jsme (just wrong, never acceptable). Learn to produce bychom; learn to merely understand bysme.

The fused conjunctions: kdyby… and aby…

Two conjunctions have absorbed the auxiliary, so they conjugate right along with it. kdyby is když + the auxiliary; aby is a + the auxiliary. The endings are identical to the standalone paradigm:

Person	kdyby- (if)	aby- (so that)
já	kdybych	abych
ty	kdybys	abys
on/ona/ono	kdyby	aby
my	kdybychom	abychom
vy	kdybyste	abyste
oni	kdyby	aby

Kdybys mi to řekl dřív, stihli bychom to.

If you'd told me sooner, we'd have made it. (to a man)

Zavolej mi, abychom se domluvili.

Call me so that we can arrange it.

Because the auxiliary is already inside these words, you never write a second one. Kdybych already says "if I would"; adding bych is doubling it.

❌ Kdyby bych měl čas…

Incorrect — kdyby already contains the auxiliary; never add bych.

✅ Kdybych měl čas…

If I had time…

The same colloquial -sme creeps in here too: you will hear kdybysme and abysme, but the standard forms are kdybychom and abychom. The full behaviour of these subordinators is on aby and kdyby clauses.

The reflexive fusion: by ses, by sis, kdyby ses

There is one more contraction, and it is obligatory, not optional. The 2nd-singular forms all end in -s (bys, kdybys, abys). When that -s meets the reflexive se or si, it detaches and hops onto the reflexive, giving ses and sis. You never write bys se or kdybys se.

Underlying	WRONG	CORRECT
bys + se	❌ bys se	✅ by ses
bys + si	❌ bys si	✅ by sis
kdybys + se	❌ kdybys se	✅ kdyby ses
abys + si	❌ abys si	✅ aby sis

Kdyby ses učil, věděl bys to.

If you studied, you'd know it. (to a man)

Přál by sis to?

Would you wish for that? (to a man)

Dej pozor, aby ses nezranil.

Be careful not to hurt yourself. (to a man)

This fusion happens only in the 2nd-person singular, because only those forms carry the stray -s. Every other person keeps the auxiliary and reflexive separate: bych se, bychom si, abyste se, kdyby se (3rd person — here the by has no -s to give away). For why these little words cluster where they do, see clitic second position.

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Quick test for the 2sg reflexive: is there an -s (from bys/kdybys/abys) AND a se/si? Then they must merge into ses/sis, and the leftover auxiliary drops its -s: kdybys se → kdyby ses. Two s-endings can never sit side by side.

Common mistakes

❌ Kde by jsi byl?

Incorrect — 2sg conditional is bys, written solid, not 'by jsi'.

✅ Kde bys byl?

Where would you be? (to a man)

❌ Rádi by jsme přišli.

Incorrect — 1pl conditional is bychom, not 'by jsme'.

✅ Rádi bychom přišli.

We'd be glad to come. (male/mixed group)

❌ Kdybys se nebál, šel bys.

Incorrect — bys + se must fuse to ses: kdyby ses.

✅ Kdyby ses nebál, šel bys.

If you weren't afraid, you'd go. (to a man)

❌ Abys si to rozmyslel.

Incorrect — abys + si must fuse to aby sis.

✅ Aby sis to rozmyslel.

So that you'd think it over. (to a man)

❌ Aby bys přišel včas.

Incorrect — aby already holds the auxiliary; the 2sg is simply abys.

✅ Abys přišel včas.

So that you'd arrive on time. (to a man)

Key takeaways

The six standalone forms: bych, bys, by, bychom, byste, by. The two to drill: bys (2sg) and bychom (1pl).
Never by jsi, by jsme, by jste — these are outright errors. Use bys, bychom, byste.
bysme is colloquial for bychom — fine in speech, wrong in formal writing.
kdyby… and aby… already contain the auxiliary and conjugate (kdybych, abys…); never add a second bych.
2nd-singular reflexive must fuse: by ses, by sis, kdyby ses, aby sis — never bys se. Two -s endings can't coexist.

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