což

1. Meaning and Function
Což introduces a non-defining relative clause that comments on, evaluates, or states the consequence of the preceding action. It signals that you are reacting to the fact of what was just said. Pedagogically, it is extremely helpful to think of což as synonymous with the coordinating conjunction a plus the pronoun to (e.g., ..., což je dobře = ..., a to je dobře). While což primarily refers to a whole clause in modern Czech, you may encounter it in older literature referring to general neuter pronouns (like to, něco, všechno), though co is overwhelmingly preferred for this today.

2. Declension (Changing Forms)
Because it refers to an abstract clause, it is inherently neuter singular. However, it must decline for case depending on its role in the new clause (often dictated by a preposition or verb). The equivalence to a to applies to these declined forms as well:

• Nominative / Accusative: což. Because these forms are identical, což can be either the subject or the object of the new clause. Czech word order or verb endings will dictate which is which (e.g., ..., což překvapilo Petra [subject] vs. ..., což Petr nečekal [object]).
• Accusative after prepositions: což (e.g., za což, na což). When referring back to a whole preceding clause, standard written Czech strictly requires o což, na což, and za což. Using contracted forms like oč, nač, or zač in this context is considered substandard, although they are perfectly standard when referring back to a pronoun (like to or něco). Note: Words like načež, pročež, and přičemž are lexicalized adverbs/conjunctions meaning 'whereupon', 'therefore', and 'while/whereby', and should not be confused with the preposition + pronoun.
• Genitive: čehož
• Dative: čemuž (e.g., k čemuž - to which / which leads to)
• Locative: (o) čemž (e.g., ..., o čemž jsme mluvili = ..., a o tom jsme mluvili)
• Instrumental: čímž

3. Syntax and Punctuation
Což (or its declined form) is typically the first constituent of its relative clause. Because it inherently carries the meaning of 'and that', it is a stylistic error to precede it with the conjunction a when it functions as a relative pronoun. If you want to use ale (but), it typically follows což as a post-initial conjunction occupying the second position (e.g., ..., což ale není pravda). The clause is always separated by a comma. If a preposition is required, the comma precedes the preposition, and the declined form of což follows it.

4. Register
Což is standard Czech and expected in writing and formal contexts. In casual, everyday conversation, speakers often use the uninflected co for the nominative/accusative. However, declined forms without -ž (like čeho, čemu) are rarely used to refer back to a whole clause. Instead, speakers naturally rephrase using a to plus the declined pronoun (e.g., saying ..., a o tom jsme mluvili instead of ..., o čemž jsme mluvili).

5. Particle vs. Relative Pronoun
Note that in phrases like A což teprve..., Což o to... or in rhetorical questions (Cožpak...), což acts as a particle expressing emotion or emphasis, not as a relative pronoun. In these cases, preceding it with a is perfectly standard.