In short

A conditional sentence links a condition (“if…”) to a result (“then…”). Polish sorts them by how likely the condition is. A real condition (it may well happen) uses plain jeśli / jeżeli plus an ordinary indicative verb. An unreal condition (it is not or was not the case) uses gdyby plus the conditional forms you met in Lesson 37. Getting the right pair - which conjunction, which verb form - is the whole skill here.

The rule

Every conditional sentence has two clauses: the condition clause (the if-clause, called poprzednik) and the result clause (the main clause, następnik). Polish groups them into two big families.

Type 1 - real / open conditions (jeśli, jeżeli, gdy)

The condition is genuinely possible. Introduce it with jeśli or jeżeli (interchangeable; jeżeli is a touch more formal), or with gdy (“when/if”) for something expected. Both clauses take the ordinary indicative - most often the future (Lessons 22-23), because a condition usually points ahead:

Jeśli + future/present indicative, (to) + future / imperative / present

• Jeśli będę miał czas, (to) przyjdę. - If I have time, I will come.
• Jeżeli pada, zostajemy w domu. - If it is raining, we stay home.

Note the tense mismatch with English: Polish says the future in both clauses (będę miał… przyjdę), where English keeps the if-clause in the present (“if I have time”).

Type 2 - unreal present/future conditions (gdyby)

The condition is contrary to fact now, or purely hypothetical. Introduce it with gdyby, which is literally gdy + by - so it already contains the conditional particle. Both clauses go into the conditional: the if-clause is gdyby + l-participle (the by lives inside gdyby and carries the personal ending), and the result clause is the full conditional (l-participle + by + ending):

Gdyby + l-participle, (to) + conditional (l-form + by + ending)

• Gdybym miał czas, (to) przyszedłbym. - If I had time, I would come.
• Gdybyś wiedziała, na pewno byś powiedziała. - If you (f) knew, you would surely tell.

Because the by is already fused into gdyby, the verb in the if-clause stays a plain l-form (miał, wiedziała) - you do not add a second by.

Type 3 - unreal past conditions

The condition was contrary to fact in the past (“if I had done…, I would have done…”). Formal/literary Polish has a dedicated past conditional (tryb przypuszczający przeszły) built with an extra był / byłby: gdybym był miał… byłbym przyszedł. In everyday modern Polish this past conditional is dropped: you use the same ordinary conditional as Type 2, and a time word or context makes it past:

• Gdybym wczoraj miał czas, przyszedłbym. - If I had had time yesterday, I would have come.
• (literary) Gdybym był wiedział, byłbym został. - Had I known, I would have stayed.

So one set of conditional forms covers both the present-unreal and the past-unreal reading; only the literary register adds był.

The resumptive “to” and word order

The result clause is often opened by to (“then”), especially when the if-clause comes first: Jeśli chcesz, to chodźmy. It is optional but very natural. Either clause may come first; when the result clause leads, drop to: Przyszedłbym, gdybym miał czas. Remember the clitic rule from Lesson 37 - by never attaches to nie: gdyby nie padało (not gdyby niby…), nie przyszedłbym (not niebym przyszedł).

Forms & patterns

The three types at a glance:

Type 2 built out - mieć “to have” in the if-clause, przyjść “to come” in the result (personal ending sits on gdyby and on by):

Where the mobile ending lands - it may ride on gdyby or hop to another early word, but it appears once per clause:

Real vs unreal, same idea:

Examples

Common mistakes

• Jeśli miałbym czas, przyszedłbym -> Gdybym miał czas, przyszedłbym - an unreal condition takes gdyby, not jeśli + by; jeśli stays with the plain indicative.
• Gdybym miałbym czas… -> Gdybym miał czas… - the by is already inside gdyby; the verb keeps a bare l-form, never a second by.
• Jeśli mam czas, przyjdę (for a future plan) -> Jeśli będę miał czas, przyjdę - the present is heard in casual speech, but standard Polish puts the if-clause in the future too when the whole sentence is about the future.
• Gdyby ja wiedziałem -> Gdybym wiedział - the personal ending fuses onto gdyby (gdybym); do not add a stray pronoun to carry it.
• niebym przyszedł / gdyby nie by padało -> nie przyszedłbym / gdyby nie padało - by never sticks to nie.
• Gdybym miał czas, przyjdę -> Gdybym miał czas, przyszedłbym - do not mix families: an unreal if-clause needs an unreal (conditional) result, not a plain future.

Tips

• Two doors, one question: is the condition really possible? Yes -> jeśli / jeżeli + indicative (Type 1). No / hypothetical -> gdyby + conditional (Type 2/3).
• gdyby = gdy + by, so the by is spent - the if-clause verb is a plain past l-form (gdybym miał), the result clause carries its own by (przyszedłbym).
• Keep both clauses in the same family: real if -> real result; unreal if -> unreal result. A future if pairs with a future/imperative result; a gdyby pairs with a conditional result.
• Modern Polish uses one conditional set for both “would” and “would have”; add a time word (wczoraj, wtedy) for the past reading and skip the literary był.
• to is your friend: dropping it in after the if-clause (Jeśli…, to…) sounds idiomatic and marks where the result begins.
• The plural virile test carries over: a group with a man -> -liby (przyszliby); women/things only -> -łyby (przyszłyby).