ledaže / pokud ne

Both pokud ne and ledaže express a negative condition or exception, translating to English as "unless." However, they handle verb polarity differently.

• Pokud ne is a neutral conditional meaning "if not." It establishes a straightforward cause-and-effect condition and requires the verb to take the negative prefix ne- (e.g., pokud nebude = unless it is / if it is not). Note that když ne is a highly frequent, slightly more casual synonym for pokud ne that follows the exact same negative-verb rules (e.g., když nebude pršet = unless it rains).
• Ledaže specifically introduces an exclusive exception ("except in the case that"). It takes the logical polarity of the exception. When translating an affirmative English "unless," it usually takes an affirmative verb (e.g., ledaže bude = unless it is), but it can take a negative verb if the exception itself is logically negative. Spelling note: ledaže is standardly written as a single word.

Direct Equivalence:
To see how the polarity difference works in practice, compare these two structures in the same context (pokud + negative verb = ledaže + affirmative verb):
Nepůjdu tam, pokud mě nepozvou. = Nepůjdu tam, ledaže mě pozvou. (I won't go unless they invite me.)

Register and Mood:
Pokud ne is highly frequent in all registers and usually takes the indicative mood for real conditions, but it can easily take the conditional mood for hypothetical negative conditions (e.g., pokud by nepršelo).
Ledaže strongly highlights an exception. It is slightly more formal or emphatic, and very frequently attaches to the conditional mood particle by (ledaže bych, ledaže bys...) to express a hypothetical exception. Remember that by is a second-position clitic and must occupy the second position in its clause.

Punctuation Rule:
Unlike in English, Czech punctuation rules require a comma to separate the clauses regardless of whether the subordinate clause comes first or second.

Important Distinctions:

• Ledaže vs. Dokud ne: Do not confuse "unless" (ledaže) with "until" (dokud ne). Nebudu jíst, dokud nepřijdeš means "I won't eat until you come."
• Ledaže vs. Leda: Leda can be a particle meaning "only" or "at most" (Mám leda stovku = I only have a hundred). However, it also acts as a conjunction, and leda by is a very common, more colloquial or informal alternative to the slightly more formal ledaže by.