aby and kdyby: Conditional Subordinators

Czech has two conjunctions that behave like nothing in English: aby and kdyby. They are not fixed little words like that or if. They have swallowed the conditional auxiliary (bych, bys, by…), and so they inflect for person — they change their ending depending on who the subject of the clause is. Once you understand that aby and kdyby literally contain the conditional inside them, their whole behavior becomes predictable.

What these words really are

Look at how they were built:

• a ("and/that") + by → aby
• když ("when/if") + by → kdyby

The by-part is the conditional auxiliary from the present conditional. Because that auxiliary inflects for person, the whole conjunction inflects for person too. This is the single fact you have to internalize.

The person lives on the conjunction — the verb is a bare participle

Here is the consequence that trips up every learner. Because the conjunction already carries the person and the conditional, the verb in the clause appears as a plain -l participle with no auxiliary of its own. The participle still agrees in gender and number, but it does not repeat the conditional.

The clause abych uspěl contains no separate bych — it would be redundant, because abych already is a + bych. Watch the conjunction change as the person changes, while the participle just agrees in gender:

aby: purpose clauses (so that, in order to)

The core meaning of aby is purpose: it answers the question why? / what for? The main clause states an action; the aby-clause states the goal that action is meant to achieve.

When the subject of the purpose is the same as the main subject and you could say it with a plain infinitive, both options exist — but a different subject forces the aby-clause. "I'm saving to buy a car" (I save, I buy) can be an infinitive; "I'm saving so that my son can study" (I save, he studies) cannot.

aby after verbs of wanting, asking, and ordering

This is where English speakers go most badly wrong. After verbs like chtít (to want), přát si (to wish), požádat (to ask), říct (to tell someone to), Czech does not use that (že) and it does not use an accusative-plus-infinitive like English I want you to come. It uses an aby-clause.

The key contrast is same subject vs different subject. If I want myself to do something, use a plain infinitive. If I want someone else to do it, the subjects differ and you need aby:

Because the person rides on the conjunction, who you want to act is encoded in the ending: Chci, aby přišel = "I want him to come"; Chci, abys přišel = "I want you to come." Getting that ending wrong changes the meaning, not just the grammar. See infinitive vs aby-clause and the conjunction aby for the full picture.

aby after verbs of fearing — with a built-in negative

Verbs of fearing (bát se "to be afraid," obávat se "to fear") take an aby-clause, and Czech adds a ne- to the verb that English does not. Bojím se, aby nespadl literally reads "I'm afraid so-that-he-not-fall," but it means "I'm afraid he'll fall." The negative is structural, not logical — don't translate it as "I'm afraid he won't fall."

kdyby: unreal and hypothetical conditions

kdyby introduces the if-clause of an unreal or hypothetical condition — something that is contrary to fact, imagined, or merely possible. It pairs with a main clause in the present conditional. The structure is symmetrical: kdyby + participle in the if-clause, bych/by + participle in the then-clause.

It is crucial to separate unreal conditions (kdyby) from real, open ones. "If it rains tomorrow, I'll stay home" is a genuine possibility, not a fantasy — that takes když plus the future, not kdyby: Když bude pršet, zůstanu doma. Reach for kdyby only when the condition is hypothetical, counterfactual, or politely tentative. The full contrast lives on kdyby — unreal conditional clauses and conditional sentences with kdyby.

A note on the comma

Both aby and kdyby introduce subordinate clauses, so Czech always sets them off with a comma — there are no exceptions. The comma comes before the conjunction: Chci, abys přišel. / Naučil bych se to, kdybych měl čas. (When the kdyby-clause comes first, the comma sits between the two clauses: Kdybych měl čas, naučil bych se to.)

Common mistakes

Key takeaways

• aby = a
  • by; kdyby = když
    • by. They inflect for person: abych/abys/aby/abychom/abyste/aby and kdybych/kdybys/kdyby/kdybychom/kdybyste/kdyby.
• The person sits on the conjunction, so the verb is a bare -l participle (gender/number only): abych přišel, kdybys věděl — never a second bych.
• aby = purpose ("so that"), and the obligatory frame after wanting/asking/fearing: Chci, abys přišel.
• kdyby = unreal/hypothetical if, paired with a conditional main clause: Kdybych měl čas, přišel bych. Real, open conditions use když
  • future instead.
• Always put a comma before the conjunction.